摘要:

Summary(i) Morphological studies relating to the origin and differentiation of the definitive germ cells in vertebrates have, as indicated, resulted in conflicting views. In many instances two or more competent investigators who have studied the same form have reached different conclusions. (2) Some contend that the germ cells are set aside from the soma during the early stages of embryonic development, and that these alone serve as the progenitors of the functional sex cells. (3) Others recognize an early differentiation of sex cells but hold that these are supplemented by others produced from the somatic epithelium of the gonad in late embryonic or post‐embryonic stages. (4) Another group recognizes the early differentiated cells as germ cells but contend that these all degenerate and that the definitive ones are formed from the germinal epithelium. These degenerating germ cells are believed by certain authors to be a phylogenetic recapitulation of the condition in lower forms. (5) Finally, yet another group contends that the so‐called primordial germ cells are not germ cells at all but are enlarged cells in some stage of mitosis or in some specific metabolic phase. This group believes that all germ cells are derived from the somatic cells of the germinal epithelium. (6) Experimental work supports the view that the primordial germ cells, which are recognized early, are the progenitors of the definitive sex cells. When these primordial germ cells are prevented from reaching the site of the developing gonad the individual fails to develop sex cells, although a sterile gonad and its associated structures may develop. (7) I suggest that the observed proliferation of germ cells from the germinal epithelium, reported by numerous investigators, can be interpreted in another way by a thorough study of the enlarged germ cells in relation to the epithelium. It seems probable that the cells of the epithelium, which form functional sex elements, are not and never were a part of the mesothelial covering, but are cells which were segregated early, and are merely stored in the epithel

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00313.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY

摘要:

Summary(i) Proceeding from a consideration of the non‐diffusible constituents of the cells, the sum of the electric charges thereon, the mean intracellularpH, and an optimum or maximum membrane potential, the inorganic levels of Na, K, H, HCO2and CI can be interpreted as physiological requirements and HCO3and CI concentrations in turn, assuming a necessary ventilation tension of CO2. (2) Omitting considerations of the membrane potential, or treating them as of secondary importance, the optimumpHvalue, and in turn the K and HCO3concentrations, are theoretically deducible by an alternative method of wider implications. (3) Including the effects of contact with the solid bone salts the concentration of the inorganic phosphate ions and minimal values for Ca and Mg ionic concentrations may also be deduce

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00314.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY

摘要:

Summary(1) Polymorphism is the occurrence together in the same habitat of two or more distinct forms of a species in such proportions that the rarest of them cannot be maintained by recurrent mutation. (2) If a genetically controlled form occurs in even a few per cent, of a population it must have been favoured by selection. (3) Polymorphism may either betransient, in which a gene is in process of spreading through a population unopposed, orbalanced, in which it is maintained at a fixed level by a balance of selective agencies. (4) Owing to the recurrent nature of mutation, transient polymorphism is generally due to changes in the environment, which make the effects of a previously disadvantageous gene beneficial. (5) A notable instance of transient polymorphism is provided by industrial melanism. (6) The explanation of this phenomenon, either on a basis of simple selection for the black forms in industrial country or of induced mutation, produced by mineral salts in the food, cannot be supported. (7) On the contrary, industrial melanism is probably due to the spread of genes improving the viability of the species, but those responsible for excess melanin production could not be used in normal circumstances, for this destroys cryptic colouring. It is now known that the genes giving rise to industrial melanism occur as rarities in the ordinary population, and that they confer a greater viability than the normal forms which they have been unable to replace. Such genes can, however, be used in industrial areas, where black colouring is a less serious handicap. (8) The variability involved in balanced polymorphism may either be environmental or genetic. (9) A balanced polymorphism generally involves a high degree of permanence in the ratios of the respective forms. (10) Variations in the relative frequencies of polymorphic forms may, however, follow cycles in absolute numbers. (11) Polymorphic forms may be distributed as clines, which may or may not lead from groups which are monomorphic. (12) Though balanced polymorphism necessitates the existence of a switch mechanism determining alternative types, these are themselves capable of genetic adjustment within the framework of the polymorphism. (13) A special kind of balanced polymorphism is associated with close linkage dependent upon sectional translocations of the chromosomes. (14) The polymorphism of heterostyled plants is due rather to the physiological than the morphological differences between the forms. (15) Sex itself is a true balanced polymorphism, environmentally or genetically controlled. (16) The human polymorphisms include the ability to taste phenyl‐thio‐urea in low concentrations and the blood groups (and conditions related to them). The relative claims of the transient and the balanced systems of polymorphisms are considered for each of t

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00315.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY