14 



J. (IB AH AM KERB. 



we have to do with an invagination of the large yolk-cells of 

 the lower lip of the blastopore by the upper dorsal lip grow- 

 ing bodily down over them. The evidence upon which this 

 belief rests is as follows : 



(a) A sagittal section through an egg of this stage fixed in 

 such a way as to avoid shrinkage of the capsule is shown in 

 outline in Text-fig. 2. It is obvious that the general outline 

 of the section suggests strongly that the dorsal lip of the 



A 



Text-fig. 3. — Camera outline of sagittal section tlirough an egg in its cap- 

 sule at a late stage of gastrulation. The lines O A, OB, and C are 

 drawn from the centre of the section so as to touch respectively the tip 

 of the archenteron (O A), the edge of the small-celled area (0 B), and 

 the dorsal lip of the blastopore (0 C). 



concave side downwards. As a matter of fact this objection is done away 

 wiih by the fact that in Protopterus frequently the lip does become concave 

 downwards just as we should expect (cf. a forthcoming paper by Mr. Budgett 

 in ' Trans. Zool. Soc. Lond.,' vol. xvi). The blastoporic lip becoming convex 

 downwards in. Lepidosiren I attribute now to the backgrowth being more 

 active in the middle line tiian laterally. 



