l6 PRINCIPLES OF EMBRYOLOGY 



choices which may become open to the material at some much later 

 period in its history, for instance whether it will differentiate in one way 

 or another during regeneration in the larval stage. In point of fact, it is 

 only in a few special cases that any difficulty arises in this connection, but 

 it is as well to bear the point in mind. 



But even though the word 'determination' may, for such reasons, often 

 need quahfication to render it fully precise, the notion still remains ex- 

 tremely important. It enables us to deal with the fact that the fundamental 

 causal happenings which control the course of development usually occur 

 long before they can be visibly recognised. They are, as might be expected, 

 chemical processes, not immediately detectable by the microscope, and 

 at present only to be discovered by testing to see whether the develop- 

 mental fate of the tissue can still be altered or not. 



In very broad outline, one may say that experimental embryology has 

 discovered three main types of mechanism which bring about determina- 

 tion. These are: 



(i) Ooplasmic segregation 



The different regions of the cytoplasm of the egg may have specific 

 properties, so that a particular region can only develop in one way. Such 

 regions are spoken of as ooplasms; an older name was 'organ-forming 

 substances'. The actual process of development depends on the occurrence 

 of some sort of interaction between the cytoplasm and a nucleus which 

 will eventually arrive in the region during the course of cleavage; but it 

 is the cytoplasm which determines the type of development. In some 

 eggs (for instance, ascidians or spirally cleaving eggs) there may be several 

 such substances; in others (for instance, Ampliibia) there may be only one. 

 Again the cytoplasmic regions may be precisely localised, with sharp 

 boundaries between them, or they may shade off into one another (as in 

 echinoderms) ; in the latter case, this type of mechanism grades into the 

 'field' type mentioned under (3). The main questions about such ooplasms 

 are, firstly, the reasons which cause them to be segregated into different 

 parts of the egg, and secondly, the nature of the interactions between 

 them and the nuclei. 



(2) Evocation 



Two neighbouring parts of an egg or embryo may react with one 

 another, in such a way as to change the capacity for development of one, 

 or perhaps sometimes of both, of the reactants. Processes of this kind 

 usually take place after the period of cleavage, when the shiftings and 

 foldings of gastrulation bring together parts of the embryo which were 



