摘要:

SummaryThe eggs of the scyphomedusae vary in size from 0–03 to 0–3 mm. diameter, a variation of a thousand‐fold in volume. Type of gastrulation is correlated fairly closely with egg size, the smallest eggs gastrulating by unipolar ingression, the largest by complete or partial invagination, while eggs of intermediate size gastrulate by a combined process of ingression and invagination. The nature of the subsequent development is equally correlated with egg size and type of gastrulation. The smallest become vermiform planula larvae incapable of constituting a polyp directly, the intermediate‐sized eggs form planulae which become polyps or scyphistomae, larger eggs may occasionally form actinula larvae and so form a polyp without having had an active planula existence, while the largest eggs develop directly into the ephyra‐medusa, omitting the scyphistoma stage entirely. In the case of the smallest, and of the smaller intermediate types, the blastula and planula stages have the capacity to grow as such without accompanying progress in differentiation. Budding may occur even at the planula stage.The scyphistoma develops four, sometimes six, gastric pouches, and forms tentacles from the rim of the oral disc, usually in the order 2, 4, 8, 12, 16, and by further addition of fours to attain sometimes as many as 32. It consists of a head and a stalk, though either part may be much the longer. Outgrowths from the body wall near the junction of head and stalk may form either pedal stolons for attachment, or buds. Buds may be liberated as free‐swimming pseudo‐planulae, may develop attached to the parent and then liberated, or may develop at the ends of long stolons. Buds of a statoblast function may be formed successively beneath the pedal disc. Tentacles of a fully grown scyphistoma may constrict off, move away as pseudo‐planulae and give rise to new polyps. There are regional differences in the capacity of pieces of the body wall to regenerate whole or partial polyps. Isolated endoderm has no capacity to reconstitute a polyp. Isolated ectoderm from any level can reconstitute a perfect whole.The type of strobilation depends upon the size and shape of the scyphistoma head or column. When the head is shallow and the stalk long, strobilation is usually monodisc, resulting in one, sometimes two ephyrae. When the head is large and more or less columnar, segmental constrictions appear, and strobilation is typically poly‐disc. Isolated segments of a polydisc strobila do not necessarily develop into ephyrae, but may reconstitute themselves into scyphistomae and become reattached as typical polyps. In polydisc forms at least, the initiation of strobilation depends upon the attainment of a large size, accumulation of food reserves, and a certain critical low temperature. Most ephyrae are liberated with eight lappets, but the first ephyra formed, that is, from the oral disc, usually has more than eight, while the last to be formed may have smaller numbers. The initial number, with their corresponding rhopalia, persists as the final number in

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00581.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Summary1. The term heterothallism was first used by Blakeslee in 1904 for the condition of sexual reproduction which he found in certain species of Mucorales, such that ‘conjugation is possible only through the interaction of two differing thalli’. In those species in which it can be shown that the thalli differ in sex, it is proposed that the term morphological heterothallism be applied, and that those species in which the unlike thalli differ by an incompatibility factor be considered to show physiological heterothallism. It is thought that the heterothallic species of Mucorales possess heterothallism of the latter type.2. Two main types of physiological heterothallism are recognized, distinguished as two‐allelomorph and multiple‐allelomorph respectively. In the two‐allelo‐morphic type, the condition is evidently determined primarily by two allelomorphs at one locus, and species possessing it are divided into strains of two kinds, commonly described as plus and minus. In the multiple‐allelomorphic type, the heterothallism appears to be determined by a multiple‐allelomorphic series at either one or two loci. If the latter, the two loci usually segregate independently at meiosis, and such species are described as tetrapolar, since each individual bears spores of four different mating types, in contradistinction to the bipolar condition of species with one locus for heterothallism.3. An outline is given of the known distribution of the occurrence of morphological heterothallism and the various types of physiological heterothallism within the fungi. Typical examples of each type are given below:I. Morphological heterothallism, e.g.Dictyuchns monosporus, Achlya bisexualis.II. Physiological heterothallism.(i) Two‐allelomorph, e.g.MucorMucedo, Ascobolusmagnificus, Sclerotinia Gladioli, Neurospora sitophila, Puccinia graminis, Ustilago Kolleri(syn.U. levis).(ii) Multiple‐allelomorph:(a)Bipolar, e.g.Coprinus comatus.(b)Tetrapolar, e.g.Coprinus fimetarius.4. Multiple‐allelomorph physiological heterothallism is shown to have advan tages over the two‐allelomorphic type in evolution, particularly if the number of allelomorphs in the population of the species at the loci for heterothallism is large. The advantages are (a) increased chance of compatible strains meeting, and (b) greater degree of outbreeding that results. The tetrapolar type of multiple‐allelomorph heterothallism gives an increased tendency for outbreeding to occur compared with the bipolar type, provided that the number of allelomorphs in the population is more than two at both loci.5. The term homothallic is applied to species in which sexual reproduction can occur in colonies derived from single spores. If such spores are uninucleate or contain nuclei of only one genotype, then the homothallism is primary. If nuclei of compatible mating types are included in each spore at the time of its formation, then the homothallism is secondary. Homothallism appears to be more frequent in Phycomycetes and Ascomycetes than in Basidiomycetes. Certain species appear to be intermediate in character between typical homothallic and heterothallic species; they may be described as showing partial heterothallism.6. In species of fungi which possess differentiated gametes or gametangia and which also show physiological heterothallism, the two phenomena are usually independent in their genetic determination and in most species have probably originated separately. Such species occasionally show unisexual strains. These appear to be mutant forms of normally monoecious species which are deficient or sterile in the organs of one or other sex.7. Heterokaryosis, or the occurrence of nuclei of more than one genotype within a single cell, is known to occur in many species of fungi and appears to be advantageous in haploid organisms. In some fungi with physiological heterothallism, heterokaryosis is restricted and can occur only between nuclei of the same mating type. In others, there is no restriction of this kind and the sex organs or gametangia are then not essential for bringing together the gamete nuclei. Herein lies a possible explanation for the degenerate condition of the sex organs or their complete absence in many species of fungi.8. Morphological and physiological heterothallism are considered to have evolved independently of one another, except in the lower Phycomycetes, and their occurrence for the most part in different groups of fungi suggests that they may have arisen comparatively rarely in evolution.9. In groups of fungi in which physiological heterothallism is manifest, it would appear on theoretical grounds that the following successive evolutionary steps must originally have taken place in the order given:(a) Homothallism is replaced by two‐allelomorph heterothallism.(b) Heterokaryosis, at first restricted to nuclei of one mating type, becomes unrestricted and the sex organs become inessential for bringing the gamete nuclei together, subsequently being partially or wholly lost.(c) Two‐allelomorph heterothallism is replaced by bipolar multiple‐allelomorph heterothallism.(d) Bipolar multiple‐allelomorph heterothallism is replaced by tetrapolar. Steps (a) and (d) are probably readily reversible, and, in consequence, existing homothallic species are probably mostly derived from heterothallic ancestors and, among species with multiple‐allelomorph heterothallism, bipolar species are probably mostly derived from tetrapolar. Secondary homothallism, which is clearly derived from a heterothallic condition, can be expected to appear at any stage after heterokaryosis has become unrestricted. The occurrence in a specific order of these various evolutionary steps provides valuable information relating to t

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00582.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

SummaryOsteoclasts, Kölliker's universal agent of bone destruction, must still be regarded as enigmatical structures. It is likely that their life span is limited to a few days. Regarded by some as resulting from fusion of relatively immobile individuals of connective tissue type, and by others of mobile cells such as macrophages, it seems a possibility that they may form from any cell, indifferently, which has assumed the ‘histiocytic state’. Osteoclasts resemble histiocytes in their reaction to supravital neutral red, in their brisk motility and in the differentiation of a superficial ecto‐plasmic layer into an ‘undulating membrane’ in tissue culture. There is no direct evidence that osteoclasts erode bone, but their constant occurrence in zones where absorption is taking place, together with the associated histological picture and especially their striated border, suggest that their presence is more than incidental. A linear orientation of cytoplasmic ultramicrons may be reflected in the striated border, which may thus constitute a cytoplasmic zone adapted for the transfer of material into or out from the cell. There is practically no evidence upon which to base an explanation of how osteoclasts could erode bone. The cells form in response to such a wide variety of normal and experimental stimuli that at least in part a local action on the bone seems likely. Local humoral factors, emanating from the bone, may be responsible for the determination of osteoclasts, and the formation of giant cells under the influence of injected phthioic acid seems to provide a valid analogy. It may be that the factors reside in the ground substance. The process of osteoclastic absorption does not appear to be regulated by the nervous system. Further experiments should be undertaken to test the ability of osteoclasts to erode bone and to ascertain whether bone about to be absorbed differs constitutionally from n

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00583.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY

摘要:

Summary1. Incompatibility is a physiological mechanism which enforces outbreeding. It is widespread throughout the families of flowering plants. There are two main types: (i)Heteromorphic.This is associated with differences in floral structure;distylicwith two types of flower, thrum having a short style and high anthers, pin having a long style and low anthers;tristylicwith three types of flowers, with long, mid and short styles. (2)Homomorphic, in which there are no floral differences.2. Heteromorphic incompatibility is associated with six contrasting pairs of characters; these are normally inherited as a single unit. The genetic control is by one gene, S, with two alleles in distyly, and by two genes each with two alleles in tristyly. The incompatibility reaction of the pollen is sporophytically and not gametophytically determined. Short style is always dominant to long. Major modifying genes and polygenes which affect the expression of the S gene are present. InPrimula hortensisabnormal plants exist in which the complex of characters has been broken down, presumably by abnormal crossing‐over between subunits of which the S gene must be composed. Both the morphological and the genetic determination restrict the number of different types, and hence of alleles.3. Homomorphic incompatibility in many species is controlled by a single gene with a large number of alleles, Sj^Sg.Sn. Pollen is unable to grow in a stylewhich has the same allele as the pollen. Unlike heteromorphic incompatibility the pollen reaction is gametophytically determined and the two alleles in the style are independent. In some species, most of which are polyploids, the simple S type of control does not work without modifications.4. In a new tetraploid incompatibility breaks down and frequently the tetraploids are fully self‐compatible. This breakdown is due to interactions between two different alleles in diploid pollen grains. With some alleles the interaction iscompetitiveso that neither allele functions. This results in self‐compatibility. With other alleles, one may be dominant to the other, thus retaining the self‐incompatibility of the diploid plant.5. Incompatibility is due in some species to the failure of pollen germination, in others to the pollen tubes failing to penetrate the full length of the style. Style‐grafting and temperature experiments indicate that the stoppage of pollen‐tube growth is due to a reaction between a substance in the pollen tube and a complementary substance in the style, the pollen and style substances being reactively different for each allele. The reaction blocks some process necessary for tube growth and is irreversible.6. Incompatibility in species crosses and their derivatives shows that species without an incompatibility system have polygenic modifiers which weaken the action of the S gene. FromNicotianaandPetuniahybrids there is evidence that the S gene can have an additional effect of inhibiting pollen from a self‐compatible species.7. The spontaneous mutation rate of the S gene inOenotheraandPrunus, including all changes, is of the order of one in a million pollen grains, but no mutations to a fully operative incompatibility allele have been found in seventy million tested grains.Mutant alleles produced by X‐rays are all of a type which have their pollen‐controlling activity destroyed but their stylar activity unimpaired. The X‐ray results and the evidence from polyploidy and interspecific hybrids all point to a dual structure of the S gene. The solution of the problem of the extremely low mutation rate with the high number of alleles in natural populations may throw light on the origin of

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1949.tb00584.x

出版商:Blackwell Publishing Ltd    

年代:1949

数据来源: WILEY