Manchester Memoirs, Vol. l. {igo6), No. ^. 57 



blastopore is represented by a blastoderm lying on the 

 surface of the yolk and including the primitive streak. 



Here again we have to urge a fatal objection. As 

 will occur to everyone, and as Keibel has already pointed 

 out, increase of yolk must take place at the vegetative 

 pole while the &gg is still in the ovary, and therefore long 

 before gastrulation can have come to pass and the yolk 

 assumed its ventral position. An increase in the yolk 

 could only delay (as in the fishes) or modify (as in the 

 Gymnophiona) the closure of the blastopore ; whereas it 

 is the already closed blastopore with which Rabl begins. 



Keibel himself has put forward a theory — which I 

 cannot discuss here — that gastrulation in the Amniota 

 takes place in two phases, the second of which is 

 expressed in the primitive groove ; rejecting Rabl's 

 hypothesis he returns to the earlier view of Balfour and 

 Duval. There is still one more suggestion, that brought 

 forward by Ziegler, which must be briefly noticed. Ziegler, 

 indeed, comes near the mark in taking the Gymnophiona 

 as his starting point, though he misses it very widely in 



In all the diagrams the (Anamnian) blastoderm is dotted, and that part 

 of its edge which becomes a blastoporic lip represented by a thick black line. 



A. The Frog. 



(i) At the end of segmentation. 



(2) When the blastopore is circular. 



(3) After rotation. 



B. Lepidosiren. 



(i), (2), and (3), similar stages. 

 There is no ventral lip. 



C. Gymnophiona. 



The yolk is increased, the blastoderm smaller, the region of its edge 

 which becomes a blastoporic lip more limited than in Lepidosiren ; but a 

 ventral lip is formed in (2) by union of the lateral lips. 



D. Amniota. 



The blastopore is formed as in Gymnophiona at the edge of the 

 embryonic shield (the inner darkly dotted area) ; the blastoderm is enlarged 

 by the addition of a surrounding belt of yolk-cells (outer lightly dotted area). 



