Manchester Memoirs, Vol. I. (1906), No. S. 17 



partly from cells carried back with the overgrowth, in part 

 from yolk-cells pushed into the segmentation cavity ; this 

 at least is the account given by Scott ; other observers — 

 Gotte, Kupffer, Shipley — have attributed it to either the 

 one or the other alone. The fate of the roof of the 

 archenteron is, however, by no means identical ; whereas 

 in the frog, as we have seen, this layer becomes differ- 

 entiated into notochord and mesoderm above, and the 

 roof of the definitive alimentary tract below, its destiny 

 in the lamprey is to give rise to the notochord and the 



Fig. 5. 

 Petromyzon (after Scott). 



A. Sagittal section after the overgrowth of the blastopore lip ; 

 d.L, dorsal lip ; arch., archenteron. 



B, C. Transverse sections showing formation of dorsal (d.m.) and 

 ventral (v.m.) mesoderm, and notochord (n.c/i.) ; m.t. (solid) medullary 

 tube. 



notochord alone. The archenteron is here a very narrow 

 cavity, and its roof proportionately small. A median 

 groove appears running along the ventral side of the roof, 

 and the whole becomes folded off and lifted out from the 

 side walls of the archenteron as a strip, subsequently a 

 rod of cells, the notochord {Fig. 5, C). The lateral walls 

 then grow in towards the middle line, and, meeting, 

 complete the roof of the gut. The endoderm, therefore, 

 does not — as far as its mode of origin is concerned — 

 wholly and exactly correspond in these two types. 



