Manchester Memoirs, Vol. I. (1906), No. 3. 55 



united by a posterior suture behind the primitive streak, 

 which suture he thought might have originated in the 

 same sort of way as the suture which connects the dorsal 

 lip with the yolk-blastopore in an Elasmobranch {Fig. 8, A). 

 The paraderm Balfour compared simply to the yolk-cells 

 or yolk of Anamnia, the upper layer to the animal cells 

 or 'ectoderm,' and made the most of the secondary con- 

 nection between the primitive streak and the paraderm, 

 whereby the necessary continuity of all germ-layers in 

 the lips of the blastopore was effected. 



Now to this theory there are two grave objections ; 



(i) The upper layer contains elements — yolk- plug 

 and floor of archenteron — which must undoubt- 

 edly be homologized with some of the yolk-cells 

 of Amphibia ; 



(2) The edge of the blastoderm in the Amniota does 

 not behave as the lip of the Anamnian blasto- 

 pore ; at it there is no rolling over of cells and 

 no formation of mesoderm. 



It may be added that the attempt made by Duval to 

 show that the primitive grove originated at the edge of 

 the blastoderm has completely broken down. The 

 blastopore is from the first moment of its appearance 

 completely within the blastoderm. 



Another solution was offered by Rabl (yFig. 32, B — D). 

 Recognising the resemblance between the Amniote primi- 

 tive groove and the circular Anamnian blastopore and 

 that the edge of the blastoderm was a new formation, 

 Rabl suggested that the increase of yolk which had taken 

 place in the transition from the Amphibia to the Protam- 

 niota had finally ended in the rupture of the body-wall 

 on the ventral side, with the result that in the Amniota 

 the body-wall of the Amphibian embryo with its circular 



