Manchester Memoirs, Vol. I. (1906), No.l^. 29 



overgrowth and ingrowth of cells, takes place {Fig. 13, A); 

 the flat epidermic layer is not included in the process. 

 This point is, of course, the dorsal lip, the ingrowing sheet 

 of cells the roof of the archenteron ;® the latter consists of 

 two or three cell layers and terminates anteriorly with 

 a free margin. The ingrowth proceeds until sufficient 

 material has been laid down for the embryo, and the 

 archenteron has reached a good length. Meanwhile, a 

 similar, though much smaller, ingrowth has been taking 



Fig. 14. 

 Serramis (after Wilson). 

 Transverse sections of the embryonic region. 



A. The roof of the archenteron {arch.) is undifferentiated. 



B. The roof of the archenteron is differentiated into notochord (n.ch.) 

 in the middle line, and mesoderm [mes.) and endoderm [end.) at the sides. 



place round the whole of the periphery of the blastoderm ; 

 this portion is merely extra-embryonic mesoderm {Fig. 

 13, B, C). When the process has, with the formation of 

 the embryo, come to an end at the dorsal lip, the 

 remainder of the edge of the blastoderm continues to 

 grow with its lip-like edge over the surface of the yolk 

 until this is completely enveloped ; the anterior edge, 

 however, travelling faster than the lateral portions, sweeps 

 round the vegetative pole and appears behind the embryo 



^ Wilson inclines to the view that the roof of the archenteron is pro- 

 duced by delamination as the thickened dorsal lip grows backwards. 



