402 PRINCIPLES OF EMBRYOLOGY 



might be, however, that this is caused not by their rarity in other forms 

 but by factors which make their detection particularly difficult. It is clear, 

 for instance, that if plasmagenes were to play an important part in the 

 differentiation of multi-cellular organisms, they could not in general be 

 capable of easy infective transmission from one cell to another, since that 

 would lead to an intermingling of different organs or types of tissue which 

 should remain separate. Thus we camiot expect to fmd many cases similar 

 to that of Billingham and Medawar, even if factors of an essentially 

 similar nature are widespread. It is necessary, therefore, to approach the 

 matter to some extent from an a priori point of view to try to determine 

 how far plasmagene-like factors could fit in to the mechanisms of 

 differentiation in so far as we understand them at present. 



It is clear that the exogenous factors mentioned under group (i) above 

 do not come into the question. In the examples of the true plasmagenes 

 mentioned in group (2), the cytoplasmic determinant is a part of the gener- 

 al genetic constitution of the organism and no more directly related to the 

 regionalisation of its various parts than are the nuclear genes. It is, however, 

 possible to imagine that the cytoplasm of the egg of a given species might 

 contain a number of different true plasmagenes localised in various regions. 

 Each region of the egg would then contain characteristic cytoplasmic 

 factors endowed with genetic continuity which might determine the 

 nature of the organs which develop out of it. Such localised plasmagenes 

 would, in fact, be the same thing as used to be referred to at the beginning 

 of this century as organ-forming substances. Now there is no doubt that 

 in many eggs different regions of the cytoplasm have different properties. 

 The regions concerned are nowadays referred to as ooplasms, and opinion 

 has rather moved against attributing their properties to the presence of 

 substances which are autonomous over against the nucleus. 



The arguments which have swayed opinion against the old idea of 

 organ-forming substances are numerous. One is that the evidence suggests 

 that the ooplasms are only effective when they are able to interact with the 

 nuclei. For instance, the cytoplasmic formation centre in the posterior of 

 an insect egg only becomes active when nuclei reach it (p. 125) ; the same is 

 true of the grey crescent ooplasm of the amphibian egg (p. 149). Again, 

 differentiation from the egg to the final form takes place in a series of steps. 

 It does not look as though we are dealing merely with the sorting out of a 

 number of factors which from the beginning preserve their character un- 

 changed, but rather as if development consists of a series of reactions dur- 

 ing which the constituents of the system change continuously until the 

 final condition is gradually built up. We are already faced with the 

 difficulty of accounting for this progressive series of changes in a system 



