SECTION II 

 DEVELOPMENT AND HEREDITY 



THERE is perhaps no phenomenon which is so impressive as the 

 development from a minute speck of protoplasm, the fertilised egg, of 

 an individual partaking of the minutest characteristics of both its 

 parents. An egg-cell has much the same appearance whether it 

 belong to an echinoderm, a fish, or a man. In the process of develop- 

 ment, by a simple repetition of a series of cell divisions, this undif- 

 ferentiated protoplasm is formed into the complex organs with the 

 potentialities and habits which distinguish the type from which the 

 protoplasm has been derived. We cannot wonder that the intimate 

 nature of this process has been the subject of speculation from the very 

 birth of science. Eunning through these speculations are two main 

 ideas, which have been labelled the theories of ' evolution ' and of 

 ' epigenesis.' By the ' evolutionists ' the egg was believed to con- 

 tain an embryo fully formed in miniature, as the bud contains the 

 flower or the chrysalis the butterfly. Development was therefore only 

 the unfolding of something already existing. If, however, this theory 

 be pushed to its utmost and if the egg contain a complete embryo, this 

 must itself contain eggs for the next generation, and so on ad infinitum, 

 a conclusion which is of course absurd. According to the theory of 

 epigenesis, the structure of the egg is wholly different from, that of 

 the adult, its development consisting in the continual formation one 

 after the other of new parts previously non-existent as such. There 

 is no doubt that this view is fundamentally correct. The difficulty 

 with which we have to contend is the understanding of the orderly 

 sequence and correlation of the cell divisions and differentiations 

 which result in an adult individual of the same type as the parents. 

 The fact that, under approximately identical conditions, one mam- 

 malian ovum will give rise to a mouse and the other to a man indicates 

 that there must be some difference in structure, organisation, or com- 

 position of the primitive egg-cell in each case, and the theory of ' evolu- 

 tion ' has reappeared in latter days in a somewhat modified form, 

 according to which the differentiation of the ovum is causally con- 

 nected with a preformed differentiation in the nuclear structures, e.g. 

 chromosomes, of the ovum itself. We have already seen that the 

 germ-cells in some types are separated off from the rest of the embryo 



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