Manchester Memoirs, Vol. I. (1906), No. 3. 27 



leads to the production of a blastoderm lying upon, but 

 not continuous with, the yolk. In the latter are numerous 

 nuclei derived from the blastoderm in an earlier stage, 

 and, as a rule, from its edge, though occasionally 

 (Salmonidae) from the whole of its lower surface. These 



Fig. 12. 



Formation of the embryo and closure of the blastopore in Semmis 

 (after Wilson). 



A, B. The material for the embryo is laid down by overgrowth at the 

 dorsal lip [d.l.) A, from the left side ; B, from above ; s.c, segmentation 

 cavity ; o.g., oil globule ; a.c, anterior edge of the blastoderm. 



C, D. Final envelopment of the yolk by growth mainly of the anterior 

 edge, or ventral lip {v. I.). 



The shading at the edge of the blastoderm indicates the formation of 

 extra-embryonic mesoderm. 



nuclei lie in a superficial clear layer of the yolk, the 

 periblast ; they do not in any wise contribute to the 

 formation of any part of the embryo, but subserve simply 

 the liquefaction and elaboration of the yolk. 



