1 6 jE'NKl'^^iSON, Gerjuifia/ Laj/ers of Vertebrates. 



To sum up, the ectoderm of the Frog is derived from 

 the cells of the animal hemisphere, the endoderm is 

 derived partly from the intermediate cells, partly from the 

 yolk-cells of the vegetative hemisphere ; while mesoderm 

 and notochord have a similar double origin. It is 

 during the closure of the blastopore — the subequatorial 

 bounding line between the animal and the vegetative 

 cells — that all three layers are brought into their definitive 

 positions, and this closure is bilaterally symmetrical, taking 

 place most rapidly at the dorsal lip. A similar statement 

 may be made of the process of germ-layer formation in 

 the remaining Anamnia. 



CYCLOSTOMATA. 

 Petromyzon. 



The Q^% of the lamprey, like that of the frog, is micro- 

 lecithal and holoblastic. At the close of segmentation it 

 presents essentially the same characters, namely, an animal 

 hemisphere of small cells, which form a roof, and a vege- 

 tative hemisphere of large yolk-cells, forming a floor for 

 the excentric segmentation cavity. 



The dorsal lip of the blastopore now appears and the 

 blastopore closes bilaterally as in the frog ; we have not, 

 however, in the case of the lamprey that exact knowledge 

 of the angle traversed by the dorsal lip. There is further 

 a very important difference between the two in that here 

 no ventral lip is formed {Fig. 5, A). Apart from that the 

 result appears to be closely similar in the two cases. The 

 overgrowth and ingrowth of cells produces an archenteron 

 which is said to open into the remains of the segmentation 

 cavity, and the material for the three layers is at the 

 same time brought into position. The ectoderm in its mode 

 of origin resembles that of the frog ; the mesoderm seems 

 here also to be derived from two sources {Fig. 5, B), 



