ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00921.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

摘要:

Summary1. In avoiding certain inherent indeterminacies in classical morphological methods and in obtaining further details regarding the microscopic and ultra‐microscopic structure of nerve axon sheaths, the methods of polarization optics and X‐ray diffraction are of great value. In the case of the myelin sheaths of vertebrate nerve fibres, for example, the optical and diffraction studies indicate the structure of the living fibre's sheath to be of smectic mixed fluid‐crystalline nature.The structure is, therefore, readily altered by chemical treatment to form the artifacts commonly observed in histological preparations.2. A number of considerations suggest that the specific configuration of the lipoid and protein components of the myelin sheath is as follows. The proteins occur as thin sheets wrapped concentrically about the axon, with two bimolecular layers of lipoids interspersed between adjacent protein layers. While this means that in a radial direction within the cylindrical sheath there are alternate predominantly aqueous and predominantly hyirocarbon phases, the latter cannot be described as being entirely “non‐aqueous”3. Polarization optical studies show that, contrary to the general view, invertebrate nerve fibres quite widely possess, aside from connective tissue investments, thin sheaths which are essentially similar in ultrastructure to the well‐defined myelin sheaths of vertebrate fibres. The demonstration of this fact involved a reinterpretation of the meaning of Gothlin's metatropic reaction, in which immersion of the fibre in media of high refractive index permits the (intrinsic) birefringence of lipoids present in the normal sheath in an oriented condition to become apparent by the reduction of the masking (form) double refraction of protein. Associated with the invertebrate metatropic axon sheaths are cells similar to the Schwann cells of vertebrate fibres.4. Quantitative birefringence studies have disclosed that the axon sheaths of a wide variety of fibre types differ chiefly with respect to the relative amounts of oriented protein and lipoid present. This difference is observed not only between typical invertebrate and vertebrate fibres, but also when the fibres of a single vertebrate nerve are compared. For example, the curve obtained when sheath birefringence of frog sciatic fibres is plotted against fibre diameter shows wide variations in the magnitude of double refraction, changing continuously from birefringence due preponderantly to lipoids, in the case of the larger fibres, to that which, in the smallest fibres, results primarily from proteins. The transition from lipoid to protein predominance occurs at a fibre diameter of about 2μ., agreeing well with the division between “medullated” and “non‐medullated” fibres arrived at by histologists. It has been suggested that the low concentration of lipoid in the sheaths of small fibres is related to physical factors opposing the introduction of the lipoids into cylindrical structures of high curvature.5. Examination of available information with respect to the relation of the velocity of impulse propagation to certain fibre characteristics, such as diameter and sheath ultrastructure, indicates that in a wide variety of fibres conduction velocity is a function of both of these factors. Thus, if fibres from invertebrate and vertebrate sources are classified according to sheath composition and ultrastructure, it is found that, within a group having similar sheaths, fast conduction is favoured by large diameter, while between groups with different sheaths, heavy myelination results in faster propagation. Comparison of fibre velocities with diameter alone, without regard to degree of myelination, is apt to be confusing, a fact which should be borne in mind in attempting to relate conduction velocity to diameter in a nerve, such as the frog sciatic, which contains fibres with very different sheaths.6. Several types of invertebrate and vertebrate unipolar ganglion cells have been observed to possess investments similar to the axon sheaths and continuous with the latter. The entire surface of these neurons, therefore, is provided with a characteristic lipoid‐protein covering, except possibly at the nodes of the myelin sheaths of the vertebrate sensory axons. The limiting envelopes of certain other cells and nuclei have been shown to possess an ultrastructure similar in type to that of the axon sheath. Permeability studies on cells have indicated the importance of lipoids and proteins in determining the properties of the plasma membrane, but it cannot be concluded that the visible envelopes are identical with the membrane which determines the physiological properties, since electrical and chemical studies favour the view that this membrane is extremely thin. The parallelisms observed between nerve sheath ultrastructure and physiological function, however, suggest some relation of these to membrane phenomena, and it is particularly difficult to understand how a multilayered structure, such as the vertebrate axon's myelin sheath, could fail to influence the chemical and electrical

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00922.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00923.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

摘要:

Summary1. Investigations on the cause of the failure of winter plants to ear in the sowing year have led to a theory of plant development based upon a discrimination between growth and development. The latter is regarded as a sequence of qualitative changes or phases, each consisting of complex internal readjustments which lead to eventual reproduction. The developmental phases proceed in a strict rotation and a subsequent phase cannot begin until the preceding phase has been completed.2. Although growth has no causal connexion with development, the true relationship between development and growth has not been conclusively investigated, while some circumstantial evidence suggests that development can be maintained only with a certain minimum of growth.3. As development is at least relatively independent of growth, the embryo can be induced to develop at a much reduced growth rate before sowing and even before complete seed ripeness. This is known as vernalization. Although vernalization has no direct effect on the vegetative period after sowing, subsequent development proceeds under relatively different conditions. The effect of vernalization is thus analogous to that of a change in time of sowing.4. The first three developmental phases and a possible transitional phase between the first two have already been established; the first two and the transitional phase being connected with the initiation of floral organs and the third with gametogenesis.5. For completion of each of the developmental phases a different but definite complex of environmental factors is required. It is essential to discriminate between factors which do or do not affect the progress of a phase. In addition, among factors which affect progress, those which are indispensable for a phase must be distinguished from those which affect the progress of a phase only in the presence of indispensable factors.6. For vernalization of the first phase a definite balanced complex of temperature, moisture and aeration is required by a physiologically potent embryo. This complex varies widely within and between species. The effect of light and darkness has not been convincingly demonstrated, although the indirect effect of day length is not excluded, particularly for a plant which is distinct from the embryo in a seed as regards the mode of nutrition. A critical thermoperiod can be established, below which the vernalizing temperature becomes ineffective.7. The complex of factors required for the second and subsequent phases has not been fully studied. In environmental studies particular attention must be given to the optimal blend of day length and temperature. Long‐day plants require light to complete the second phase and can only tolerate darkness if the latter is not in excess, while short‐day plants show the opposite relation. The darkness requirement of short‐day plants can be deciphered as an inhibitory effect of light upon the second phase, if the intensity of light is above a definite maximum. The relation of cereals at the third phase seems to be in conflict with Lysenko's ecological concepts.8. The physiology of plant development has received little study. Not all the changes pertaining to development are detectable morphologically. The changes constituting a phase are gradual, quantitative, additive and irreversible, and their completion causes a qualitative change in the internal environment of a plant. Thereby the properties and relation of a plant to its environment are also changed.9. The changes pertaining to an advance in development are elaborated and retained in the promeristem and transmitted only through cell division, further elaboration taking place in the daughter cells. The entire body of the embryo is sensitive to temperature, whereas the response of the promeristem to photoperiods is effected through some activities in the leaves. In the latter a catalytic substance is secreted, which on being transmitted to the tip participates in the developmental processes.10. The nature of the changes constituting the first phase has received little attention. The hormonal theory of vernalization is inconsistent with the theory of phasic development and has not been substantiated. Of the enzymes studied, only catalases and peroxidases were suggestive. Presumgbly the enzymes and hormones are concerned more closely with the rate of growth than of development. The endosperm, aleurone layers or integuments do not participate directly in the developmental process, which is confined to the embryo alone. Investigations of the physical and chemical changes in the protoplasm in relation to phasic development were more suggestive and provided a method of diagnosis of the first two phases. The nature of the second and subsequent phases has not been studied.11. In environmental studies the biochemical method of diagnosis should be employed whenever possible, as the conceptions based upon the “after‐effect of vernalization” are not always reliable. The possibility of vernalizing the embryo during seed ripening has necessitated a thorough revision of the conceptions hitherto formed regarding the length of developmental phases and earliness or lateness in various ecotypes.12. The genetical conceptions announced by Lysenko differ but little from the orthodox conceptions, greater stress having been laid on the effect of the environment. The cytogenetical conceptions require further study, while the changes in the genotype of a plant induced, as it is claimed, as a result of adaptation to a new environment (training of plants) must be studied cy

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00924.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00925.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY

摘要:

SummaryWhen skeleton‐forming cells of a donor sea urchin embryo are transplanted into a host embryo of another species, whose endoderm and mesoderm have previously been removed so that it only possesses ectoderm, a larva may be produced which is a chimaera consisting only of skin and skeleton. The donor skeleton is harmoniously situated in the host larva, thanks to the influence of the latter. But the skeleton affects the host inasmuch as it forces the latter to form larval processes. The skeletal structure is of the donor type.When skeleton‐forming cells are implanted into the whole larva of another species, an intermediate type of skeleton arises, with the exception of the skeletal rods, which occur only in the host form. These are developed exactly as in the host form.When skeleton‐forming cells are implanted into an embryo whose own skeleton‐forming cells have previously been removed, a skeleton develops which at first has the donor structure. Later on the host also supplies skeleton‐forming cells, and the skeleton which has already been formed gradually changes towards that of the host form.Hybrids obtained by cross‐fertilization of the same forms as those which made the chimaeras also have intermediate skeletons.When a species‐hybrid is made (A ♀×B ♂), its skeleton‐forming cells contain only maternal cytoplasm (A), but half maternal (A) and half paternal (B) chromatin. When the skeleton‐forming cells of such a hybrid are implanted into an embryo of the maternal species (A), whose skeleton‐forming cells contain bothAcytoplasm andAchromatin alone, a hybrid chimaera is obtained, the skeleton‐forming cells of which contain cytoplasm of the maternal species (A) alone but chromatin of both species in the ratio of 3A: I B. The skeletons are intermediate, but approach nearer to the maternal type. If the maternal component is weakened by the excision of some skeleton‐forming cells from the host before the implantation, then the skeleton is more definitely intermediate.The formation of a skeletal rod depends on two factors, the presence of the arm‐ectoderrn and of the corresponding skeleton‐forming cells. If the arm‐ectoderm is absent, the corresponding skeletal rod cannot be formed. If the arm‐ectoderm is present, and the skeleton‐forming cells are hybrids between a species which normally possesses a skeletal rod and one which lacks it, then the rod is not formed. Thus the absence of the skeleton‐forming factor is dominant to its presence.A study of normal skeleton formation gives the impression that the skeletogenous cytoplasm ofEchinocyamushas a lower viscosity than that ofPsammechinus. Protoplasmic viscosity seems to be one of the factors determining the particular structure of the skeleton.The fact that in sea‐urchin larvae with simple skeletons there appear “directed variations” tending towards the type of the more complicated forms is explicable in this manner. It was found to be possible, through the effects of high temperature and chemical substances, to influence larvae ofEchinusvery considerably in the direction ofEchinocyamus.The intermediate nature of hybrids is due to the fact that the nuclei of the skeleton‐forming cells contain elements of both parental species. These nuclei control the viscosity of the skeletogenous syncytium in which they lie.The intermediate nature of chimaeras is due to the nuclei of both species being placed in a mixed syncytium belonging to both species, which consequently has an intermediate viscosity.The intermediate nature of variants is due to the viscosity of the skeletogenous

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1939.tb00926.x

出版商:Blackwell Publishing Ltd    

年代:1939

数据来源: WILEY