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 crescentic 

 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 



coelenteron 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 

 germ-layers. 



The points of 

 comp arison 

 with the gas- 

 trula of Triton 

 (fig. 47) are 

 made evident 

 as soon as we 

 replace the 

 mass of yolk- 

 cells 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 

 Birds 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, RAUBER, DUVAL, and others), the lower layer has arisen by 



\ 



Fig. 55. Embryonic fundament of Lacerta agilis, after KUPFFER. 

 If, Area pellucida ; df, area opaca ; , blastopore ; s, crescent ; es, em- 

 bryonic shield. V, anterior, H, posterior end. 



