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



33 



floor of the archenteron, grow up beneath the mesoderm, 

 and, fusing in the middle hne below the notochord, 



Fig. 18. 

 Ceratodus (after Semon). 



A. Transverse section, showing wide archenteron. 



B. The plate of cells forming the roof of the archenteron is being 

 undergrown by yolk-cells from the sides. 



Lettering as above. 



complete the dorsal wall of the alimentary canal {Fig. 

 18, B). 



In Lepidosircn on the other hand, the o.^^ inclines 

 towards a meroblastic segmentation. The blastopore 



Fig. 19. 

 Lepidosircn (after Graham Kerr). 



A. Sagittal section during overgrowth of dorsal lip {d.l.), and forma- 

 tion of archenteron {arch.). Note that there is no ventral lip. 



B. Transverse section, showing dorsal mesoderm (we-j.) brought into 

 position with the overgrowth of the lip of the blastopore, and notochord 

 {n.ch. ) split off the roof of the archenteron. 



