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 development, by a simple repetition of a series 

 of cell divisions, this undifferentiated 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. Running through these speculations are two main 

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

 genesis.' By the ' evolutionists ' the egg was believed to contain 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 mammalian ovum will give rise to 

 a mouse and the other to a man indicates that there must be some difference 

 in structure, organisation, or composition of the primitive egg cell in each 

 case, and the theory of ' evolution ' has reappeared in later days in a some- 

 what modified form, according to which the differentiation of the ovum 

 is causally connected with a preformed differentiation in the nuclear struc- 

 tures 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 at a 

 fairly early stage. If, in the two-celled stage of the frog's egg, one cell be 

 destroyed by means of a hot wire, the other cell develops to form half an 

 embryo, thus suggesting that each cell of the two-celled embryo could give 

 rise only to the corresponding half of the body. This limitation of develop- 

 ment however occurs only if the intact cell be left in connection with the 



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