摘要:

Summary1. The relative low prolificacy of the ewe is a factor severely limiting the sheep's ability to compete with other forms of livestock production in intensive agriculture. This article is an attempt to examine critically all the available information on the reproductive physiology of the ewe, with particular emphasis on the factors controlling the various phenomena and the possibility of increasing prolificacy.2. All breeds exhibit, to a greater or less extent, a breeding season which is under light control. Available data on the extent of this season are tabulated.3. The ‘height’ of the breeding season corresponds roughly to the shortest day of the year, and the ‘depth’ of anoestrum to the longest day.4. This and other data suggest a seasonal fluctuation in pituitary gonadotrophic activity controlled by, and roughly inversely related to, the period of daylight relative to dark. It reaches a maximum when the days are shortest and a minimum when they are longest.The ability of some breeds to come into oestrus and conceive shortly after parturition depends on the breeding season being sufficiently long to permit ewes mated early in the season to lamb well before its end. Provided then that the lactation anoestrum, the length of which appears to vary according to the stage of the breeding season, does not run into the true anoestrum the ewes can mate and conceive again while lactating.6. The onset of the breeding season is preceded by an ovulation without heat–‘silent heat’. The evocation of heat with ovulation appears dependent on the presence of a waning corpus luteum.7. Seasonal variation in the reproductive capacity of the ram is evident but is not as marked as in the ewe.8. The literature on the dioestrous cycle, ovulation, fertilization, attachment and subsequent development of the embryo and foetus is reviewed. Very loose attachment to the cotyledons occurs at about 11 days after ovulation, which attachment remains loose up to the 30th day. Full placentation is not completed until the middle of pregnancy.9. Embryonic mortality within the first 21 days accounts for an appreciable number of the ova fertilized. While the number of ova shed is probably the main factor controlling the number of lambs born, this mortality must be considered.10. The use of gonadotropins in anoestrum to obtain early pregnancies is fully discussed and the literature reviewed. While, in general, results have been disappointing there appears to be a definite pattern of behaviour depending on the state of the ovaries at the time of injection.11. An interesting report on the use of pregnant mare's serum (PMS) in inducing fertile matings shortly after parturition within the breeding season is discussed. If this can be confirmed, it may be of value in obtaining two pregnancies within a year.12. The subcutaneous injection of 500 i.u. of PMS 12 days after the preceding oestrus will result in multiple ovulations at the ensuing heat period 4 or 5 days later. However, the individual response to any given level of PMS is extremely variable, from two to nine ovulations having been observed following 500 i.u. Mating at this oestrus results in a high proportion of fertilizations and attachments. Subsequent embryonic mortality within the first 21 days following ovulation results in a reduction in the number of embryos surviving to two or three, occasionally four and rarely five or six. Up to four live young have been obtained at Cambridge and five in Russia. The number of lambs born in a small group of Suffolk ewes treated at Cambridge was greater than in the control group. However, some were born dead and the number weaned was similar in both groups.13. There appear to be breed differences in the number of embryos surviving the first 3 weeks of pregnancy following such PMS treatment, probably due to differences in uterine environment.14. Overstimulation with gonadotropins results in accelerated tubal transport of the ova, with lack of fertilization and complete failure to attach.15. Taking a long‐term view, increase in prolificacy can probably be best obtained by selection for a long breeding season and for multiple births. In most breeds there is sufficient variation in these characteristics to offer considerable

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1951.tb00644.x

出版商:Blackwell Publishing Ltd    

年代:1951

数据来源: WILEY

摘要:

SummaryThere are four methods of analysing the vertical migration of plankton in the field:(a)One day and one night observation at the surface.(b)One day and one night observation at depths between surface and the bottom.(c)Observations at long time intervals at all depths during day and night.(d)Observations at short time intervals at all depths during day and night.The last method, using horizontal closing tow‐nets with flow meters at measured depths, shows that migration consists of four parts:(a)ascent from the day depth;(b)midnight sinking;(c)dawn rise;(d)descent to the day depth.This pattern has been shown to be modified by wind (or state of sea), rain, sun on a calm sea, extremes of temperature, age of the individual animal and possibly by phytoplankton concentrations.These different parts of migration have been examined in detail and linked together in a single and continuous process. Ascent in the evening and descent in the morning become the continuous change of day depth with the changing penetration of light; the midnight sinking is assumed to be due to a passive state in full darkness; the dawn rise is a return by the animals to the mean optimum light intensity for the population. This is supported by the fact that the order in time of arrival at the surface for some fresh‐water species is the same as the order in depth of these species in full daylight.Theoretical interpretations of migration are reviewed. It is argued that the most likely theory has as its basic assumption the idea of an optimum light intensity.Experiments on migration are divided into three parts:(a)those showing that randomly oriented movement can take place;(b)those showing that the speeds of individual animals can vary with varying light intensities;(c)those showing that animals in full darkness will move at minimal speeds or not at all.It is concluded that the vertical migration of planktonic Crustacea is mediated by the change in light penetration throughout the day. Animals aggregate in an optimum band of light intensity. They have the capacity of moving phototactically, i.e. moving at constant speed towards or away from light; they also have the capacity of moving photokinetically, i.e. moving at speeds that vary with varying light intensity. In full darkness they move probably at minimal speeds. Migration between periods of full darkness may be operated by phototaxis, photokinesis or by a combination of these mechani

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1951.tb00645.x

出版商:Blackwell Publishing Ltd    

年代:1951

数据来源: WILEY

摘要:

Summary1. Current theories of ophidian evolution suggest that the snakes have been derived either from aquatic, above‐ground terrestrial, or burrowing ancestors. It is generally supposed that the ancestors of snakes, irrespective of their habits, were closely related to the platynotid lizards. In the absence of adequate palaeontological data, the respective merits of these assumptions must mainly be assessed on evidence obtained from study of living forms.2. The close relationship between the snakes and lizards is pointed out in this article, and the main characteristics of the different families of fossil and living snakes are reviewed.3. Many important features of the snakes as a group are described and their functional significance is discussed. Comparison with the corresponding condition in lizards is made. Ophidian characters regarded as being of particular significance include the bony investment of the forebrain, the platytrabic chondrocranium with absence of the interorbital septum and endocranial side‐walls, the absence of the epipterygoids and temporal arcades and the extreme mobility of the jaw skeleton in most forms; the presence of zygosphenal articulations throughout the vertebral column; the elaboration of the trunk musculature and ventral scales; the extreme concentration of alkali‐metal cation on the secretory side. (For reviews of evidence, see Crane (1950), Hokin&Rehm (1947), Rehm (1950).) (2) When the two sides of the mucosa are connected electrically, a current of relatively large magnitude can be maintained which requires the continuous performance of electrical work (Rehm, 1943; Rehm&Hokin, 1948; Crane, Davies&Longmuir, 1946, 1948a; Crane&Davies, 19486). (3) There is a fall in the p.d. across the mucosa with the onset of secretion, but once secretion is well established further increases in the rate of secretion can occur with only small changes in the p.d. (cf. Crane, 1950; Rehm, 1944; Rehm, 1950). (4) The change from the non‐secretory to the secretory state and back is always associated with concomitant changes in the electrical parameters of the mucosa, whether these changes occur spontaneously or by drug action. In the absence of oxygen and in dead mucosae both p.d. and acid secretion are abolished (cf. Crane, 1950; Rehm&Hokin, 1947; Rehm, 1950). (5) Passage of electric current(c.1 mA./cm.2) through the mucosa from an external source increases or decreases the rate of secretion of acid by a secreting mucosa according as the applied current enhances or opposes the spontaneous potential measured between calomel electrodes on either side of the mucosa (Rehm, 1945; Craneet al.1946, 19486). (6) Passage of current does not initiate secretion in the dog (Rehm, 1945) but may do so in the frog (Craneet al.1948b).These results were obtained with intact gastric mucosa, and because of the technical difficulties involved it has not, so far, been possible to obtain direct information regarding the detailed electric changes in and around the oxyntic cells. A difficulty is that these cells are mostly isolated from each other and make up only about one‐tenth of the volume of the mucosa (Davies&Roughton, 1948) and about one‐twentieth of the dry weight (calculated from Engström&Glick, 1950). However, quantitative relations have been estimated (Rehm, 1943; Rehm&Hokin, 1948; Rehm, 1950; Crane&Davies, 1948b, 1950b), and it seems certain that the energy required for secretion by acid‐secreting gastric mucosa is greater than the maximum electrical energy output of non‐secreting gastric mucosa.Since acid secretion is associated with a great increase in the rate of respiration of oxyntic cells (Davies, 1948a), and hence in the available metabolic power, care is clearly needed when comparing activities in the secreting and non‐secreting mucosa.It is more difficult to determine the relation between the energy required for the increase in acid secretion during the passage of electric current from nutrient to secretory side, and the energy available from this current. Although the total power expended by the current is greater than that required, the fraction capable of doing useful work is probably less. On the other hand, it seems probable that the gastric mucosa can supply sufficient coulombs of electricity to account for the production of the hydrogen ions, and that this also applies to the increased rate of acid secretion when current from an external source is passed through the mucosa (Rehm, 1950).Before attempting to include these observations in a theory of the mechanism of hydrochloric acid production it is necessary to return to a further consideration of the biochemistry of gastric mucosa. the condition in burrowing lizards, together with the other primitive features displayed, suggest that forms such asCylindrophis(Fig. iA) andIlysiahave not departed greatly from the basal ophidian type. The late CretaceousDinilysiaseems unlikely to have been a burrower on account of its relatively large size. From the evolutionary standpoint adopted, this snake may be considered as representing an early venture in above‐ground radiation on the part of this archaic family.The Uropeltidae shows many resemblances with the Anilidae (Haas, 1931b), but has acquired certain aberrant features (e.g. modified occipito‐vertebral articulation), and the vestigial hindlimbs have undergone further reduction. The presence of two transverse ventral scale rows to each body segment (possibly a primitive feature and shared by the Leptotyphlopidae and Typhlopidae) suggests that the Uropeltidae have not been directly derived from the Anilidae.The Boidae and the allied Xenopeltidae possess many primitive characters, especially of the viscera and vascular system; at the same time they show certain advanced features and have undergone a minimum of aberrant specialization. The central position accorded to these families in Boulenger's scheme (1893‐6) appears to be fully justified. While zoo‐geographical considerations seem to show that the Boinae are the more primitive of the two boid subfamilies (Darlington, 1948), the occurrence of such features as premaxillary teeth and a supraorbital bone among the Pythoninae suggest that the latter is the older group. The whole family is of particular interest since its different members perhaps illustrate, both in structure and in range of habits, the characteristics of the snakes in general at the end of the Mesozoic, during the early stages in their radiation as an above‐ground group.Summary1. Current theories of ophidian evolution suggest that the snakes have been derived either from aquatic, above‐ground terrestrial, or burrowing ancestors. It is generally supposed that the ancestors of snakes, irrespective of their habits, were closely related to the platynotid lizards. In the absence of adequate palaeontological data, the respective merits of these assumptions must mainly be assessed on evidence obtained from study of living forms.2. The close relationship between the snakes and lizards is pointed out in this article, and the main characteristics of the different families of fossil and living snakes are reviewed.3. Many important features of the snakes as a group are described and their functional significance is discussed. Comparison with the corresponding condition in lizards is made. Ophidian characters regarded as being of particular significance include the bony investment of the forebrain, the platytrabic chondrocranium with absence of the interorbital septum and endocranial side‐walls, the absence of the epipterygoids and temporal arcades and the extreme mobility of the jaw skeleton in most forms; the presence of zygosphenal articulations throughout the vertebral column; the elaboration of the trunk musculature and ventral scales; the extreme reduction or loss of limbs; the great development of the vomeronasal apparatus; many peculiarities of the eyes and their adnexae (study of which led Walls to advance the view that the snakes were derived from forms in which the eyes had degenerated markedly in association with burrowing habits and subsequently became re‐elaborated, permitting bsence of the tympanic membrane and middle ear cavity; the elongation and asymmetry of the viscera and their blood supply; the presence of paired copulatory organs in the male.4. Comparison with lizards indicates that many of the latter show a close approach to the ophidian type of organization, although the features common to all lizards and snakes are comparatively few.5. The phylogeny and characteristics of the platynotid lizards are described, and the similarities between the different platynotid groups and snakes, both fossil and living, are discussed. While the Platynota in general, and the Dolichosauridae in particular, show many snake‐like features, they appear to differ markedly from all snakes in the structure of the skull and in certain other characters. It is suggested that the evidence is inadequate to establish close relationship between the snakes and any of the known platynotid types; the origin of snakes from unknown platy‐notids during the early Mesozoic remains a possibility.6. The very numerous resemblances between burrowing lizards and snakes suggest that the ophidian ancestors were fossorial rather than above‐ground or aquatic in habits. Some structural and biological changes which might possibly have occurred if the snakes had passed through a burrowing phase in their early evolutionary history and subsequently returned to epigean life are described. It is suggested that the potentialities for re‐elaboration of the eye, the development of a wide gape, and the perfection of the locomotory mechanism were the essential features in permitting eventual radiation of the snakes as an above‐ground group. Snakes are perhaps the only vertebrates in which loss of limbs has not been followed by restriction of adaptive possibilities.7. The affinities of the more primitive families of living snakes are briefly considered; it is suggested that the primitive burrowing snakes are to some extent representative of the early stages in the history of

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1951.tb00646.x

出版商:Blackwell Publishing Ltd    

年代:1951

数据来源: WILEY