ch. ix ONTOGENETIC EVOLUTION 485 



colours of the spectrum are present in ordinary " white " light but are 

 invisible until they are sifted out from one another by the action of 

 a prism. 



The lesson learned from the developing pelagic zygote — that in 

 its case the full equipment of the complete individual is provided 

 from internal sources— is one which should ever be borne in mind. 

 It makes it easier to realize that in other cases, where the developing 

 organism exists in a less homogeneous environment and where it has 

 to fend for itself, characters impressed upon it directly by the 

 environment, however conspicuous, are still superficial as compared 

 with the really fundamental characters already present in the 

 zygote. 



The course of ontogenetic development from the zygote stage 

 involves two main processes, (1) increase in bulk accompanied by 

 the assumption of a multicellular condition, and (2) differentiation 

 of parts i.e. the segregation, into localized portions of the living 

 substance, of peculiarities which were in the zygote distributed with- 

 out definite arrangement. The topographical differentiation of the 

 developing embryo does not necessarily keep exact pace with the 

 subdivision into cells. Thus in Amphioxus the egg appears to be 

 still homogeneous throughout up to the time when it has segmented 

 into 8 or even 16 blastomeres for even at this stage a blastomere isolated 

 from its neighbours experimentally may go on for a time pursuing 

 the same course of development as it would have done were it a 

 complete zygote. In other cases, as appears to be the rule in the 

 Frog, the first step in segregation — the segregation of characters 

 belonging to the right and left halves of the body into corresponding 

 hemispheres of the egg — would appear to take place in the zygote 

 stage i.e. before the appearance of the first cleavage furrow. 



The progressive segregation of specific characters in the various 

 parts of the developing individual is beautifully brought out in the 

 case of various invertebrates by the elaborate studies on " Cell 

 lineage," some of which have been fully described in Vol. I. 



The animal individual lives its life under a particular set of 

 environmental conditions, constituted by the external medium — 

 water or air — with its other living inhabitants : the latter play 

 an important part, it may be by such comparatively simple and direct 

 methods as by affecting the composition of the external medium, it 

 may be by far more complex and obscure influences due to biological 

 inter-relationships. The individual is able to go on living because 

 of its organization and its living activities being fitted, in the most 

 intimate manner, into the particular set of conditions which con- 

 stitute its environment. 



So also with the various parts of the body — organs, tissues, 

 individual cells— of the young developing individual. Each lives 

 amidst an environment of extreme complexity and of perfectly 

 definite type of complexity, conditioned by the nature of the body 

 of which it forms a portion and by the character of the parts of the 



