Manchester Memoirs^ Vol. I. (1906), No. 3. 59 



a small area of the outer zone of cells behind 

 which they meet and fuse to form a (virtual, or 

 in some cases an actual) ventral lip to the now 

 circular blastopore. In this process the anterior 

 margin of the embryonic shield is wholly 

 unconcerned. 



(3) The archenteron which is formed at the blasto- 



poric lip opens into the sub-germinal cavity. 



(4) Notochord and mesoderm are derived from the 



ingrowth at the lips of the blastopore, while the 

 endoderm arises from the paraderm. 

 I venture to think that, considering the difference in 

 the amount of yolk present in the two cases, a closer 

 parallelism could hardly have been expected ; viutatis 

 nmtandis the description of either might be applied to 

 the other type. 



It seems clear then that 



(i) The embryonic shield of the Amniota is the 

 representative of the blastoderm of the Anamnia. 



(2) The marginal zone of the upper layer of the 

 Amniota, together with the paraderm, represents 

 the yolk-cells or nucleated yolk of the Anamnia 

 {Fig. 32, D). 

 In passing from the Gymnophiona — which I suppose 

 are as nearly related as any living group to the ancestors 

 of the Amniota — to these higher forms, we have, therefore, 

 only to suppose that with the enlargement of the vegeta- 

 tative hemisphere of the egg segmentation has become 

 restricted, not to the blastoderm alone (as in the fishes), 

 but to the blastoderm and those circumjacent and sub- 

 jacent cells which in the Gymnophiona are partially 

 segmented from the yolk. In the most primitive Reptiles 

 the paraderm cells are still crowded with yolk, and still 



