96 



EMBRYOLOGY. 



centic groove (fig. 52 s) corresponds to the blastopore ; the thickened 

 portion of the marginal ridge (fig. 53 vl) which lies in front of the 

 crescentic groove, within whose territory the two primary germ- 

 layers are continuous with each other, is the anterior or dorsal lip of 

 the blastopore; and the yolk (hi) which lies behind the crescnitic 

 groove, and which at this early stage contains numerous free nuclei, 

 may be designated as the posterior or ventral lip of the blastopore. 



The develop- 



v ment of the 



ccelenteron is 

 the cause of 

 the gradual re- 

 duction of the 

 cleavage - cav- 

 ity, and of its 

 persisting only 

 as a narrow fis- 

 -, sure separating 

 the primary 

 y. germ-layers. 



The points of 

 ^ comparison 

 i with the gas- 

 t nil a of Triton 

 (fig. 47) are 



made evident 

 // 



as soon as we 



Tig. 55. Embryonic fundamsnt of Lacsrta agilis, after KUPFFER. , , 



hf, Area pellucida ; df, area opaca ; u, blastopore ; s, crescent ; es, em- r e P * a c e the 

 bryonic shield. V, anterior, //, posterior end. inaSS of Volk- 



c:lls with un- 



segmented yolk, and imagine nuclei imbedded in the latter in the 

 region of the ventral lip of the blastopore. 



Through the exposition given by DUVAL, it appears to me that the 

 contest concerning the origin of the two primary germ-layers in 

 Lirds has been happily settled. .For a long time there have existed 

 on this very question two irreconcilable views. 



According to the older view, to which many investigators still cling, 

 the germ-disc which results from the process of cleavage is divided by 

 fission into an upper and a lower layer (PANDER, VON BAER, REMAK, 

 KOLLIKER, His, and others). According to the other one (HAECKEL, 

 GOETTE, EAUBER, DUVAL, and ethers), the lower layer has arisen by 



