Manchester Memoirs, Vol. I. (1906), No. 3. 25 



certain. When the embryo begins to be folded off from 

 the hinder middle region of the blastoderm, the roof of the 

 archenteron is involved in the folding, and forms at least 

 the roof of the gut ; the floor, according to Balfour, is 

 derived from a layer of cells formed round the nuclei of 

 the underlying yolk. I must say that my own prepara- 

 tions do not support this view ; the yolk-nuclei are much 

 larger than, and totally unlike, any of the nuclei of the 

 blastoderm ; and it seems to me perfectly clear that the 

 gut is closed ventrally by suture of the sides of the archen- 

 teron as this becomes folded o^ {Fig. 11, A, B). 



Up to the present it is the posterior edge or dorsal 

 lip which has been principally active, but now growth 

 ceases at that point, elongation of the embryo being 

 effected by the independent proliferation of the caudal 

 swellings {Fig. 7, F). The anterior and lateral margins of 

 the blastoderm, on the contrary, become exceedingly 

 vigorous and begin to grow over the yolk, the overgrowth 

 being accompanied, as stated above, by a slight marginal 

 invagination ; and eventually the anterior edge makes 

 the whole circuit of the yolk, passing round the vegetative 

 pole and reappearing behind the embryo as the ventral 

 lip of the small ' yolk-blastopore ' {Fig. 8, B — D). A 

 peculiar change has also taken place on each side of the 

 dorsal lip, the lateral lips immediately adjacent to it 

 having swung back until they bound a narrow median 

 strip of yolk {Fig. 8, A), by which alone the aperture at 

 the dorsal lip now communicates with the remainder of the 

 blastopore. 



It is now possible for us to institute a series of com- 

 parisons between the processes we have observed in the 

 microlecithal and macrolecithal types. These com- 

 parisons are as follows : — 



(i) The blastoderm of the Elasmobranch corresponds 



