32 Jenkinson, Germinal Layers of Vertebrates. 



is intermediate between the holoblastic and the mero- 

 blastic types : the yolk becomes but very slowly divided. 



The closure of the blastopore — during which the 

 material for the notochord and mesoderm is brought into 



1/ 



Fig. 17. 

 Formation of the germ-layers in Amia (after Sobotta). 



A. Axial section of the egg at the close of segmentation ; the yolk only 

 partially segmented. 



B. Sagittal section after closure of the blastopore. Letters as before. 



position and the archenteron formed — is bilaterally 

 symmetrical, exhibiting the usual preponderance of over- 

 growth at the dorsal lip. The blastopore becomes com- 

 pletely circular, and is provided with a ventral lip 

 {Figs. 16, 17). 



DIPNOI. 



With regard to the Dipnoi we are, however, thanks to 

 the researches of Graham Kerr and Semon, better 

 informed. 



In Ceratodiis the &^^ is holoblastic. Closure of the 

 blastopore is bilaterally symmetrical, there is a ventral as 

 well as a dorsal lip, and a spacious archenteric cavity is 

 formed, which does not communicate with the segmenta- 

 tion cavity. The roof of the archenteron consists of a 

 broad cell plate {Fig. 18, A) which becomes divided 

 into a median strip, the notochord, and two lateral 

 mesodermal sheets. The vegetative cells, which line the 



