248 PRINCIPLES OF EMBRYOLOGY 



proceeds to be invaginated arouiid the same focus. In birds we have no 

 direct evidence as to the mechanisms v^liich determine the posterior end 

 of the cndoderm, v^hich is the region that takes the lead as soon as the 

 dorso-ventral plane is fixed. In tliis form, however, another stage is inter- 

 polated into the normal sequence, since there is something of a pause be- 

 tween cndoderm formation and the start of invagination of mesoderm at 

 the primitive streak, and we find that the cndoderm exerts a causal influ- 

 ence wliich determines the site of mesoderm invagination. The apparently 

 straightforward, follow-my-leader behaviour of mesoderm towards cndo- 

 derm m the lower groups has, therefore, been expanded in the birds into 

 a cause-effect sequence. 



We still know too little about the epigenetics of mammals to fit them 

 with any confidence into the scheme. There is evidence (Dalcq, p. 238) 

 that the oocyte and newly fertilised egg have some degree of bilaterality 

 of structure. Seidel, as a result of his separation of blastomeres, argued that 

 there is a localised organisation centre. This result, however, was based 

 on only two sets of operations and is exceedingly tentative. On the other 

 hand, the evidence suggests that the formation of identical twins may 

 occur at much later stages; and on the whole it seems probable that in 

 mammals, as in birds, the final determination of the dorso-ventral plane 

 occurs rather late. 



It seems probable that, in all the vertebrates without exception, deter- 

 mination of the dorso-ventral plane fixes, in the first place, the location 

 of the cndoderm and, secondly, that of the mesoderm, but that the ecto- 

 derm remains quite indifferent till it is acted upon by an organising influ- 

 ence coming from the mesoderm. Grafting experiments have given direct 

 proof of the inducing power of the presumptive mesoderm in cyclostomes, 

 teleosts, Ampliibia and birds and the demonstration is only slightly less 

 clear-cut in mammals. In all groups there is evidence that at the time the 

 mesoderm starts to be invaginated the fate of particular regions of it can 

 still easily be altered, but that, as invagination proceeds, it is affected by a 

 process of regionahsation so that different areas become determined, not 

 only as to the tissue into which they will develop (as notochord, somites, 

 nephros, etc.), but also as to their position on the anterior-posterior axis 

 (brain, spinal cord, tail, etc.). There is some not very compclhng evidence 

 (p. 187) that in birds the presumptive forebrain has a tendency to develop 

 into neural tissue independently of any induction by mesoderm. If this 

 is so it may be connected with the fact that, in this group, the determina- 

 tion of the site of mesoderm formation (the primitive streak) is a com- 

 paratively long-drawn-out process caused by the cndoderm. In other 

 groups the development of neural tissue seems to be completely dependent 



