( 803 ) 



bv the formation of the groove thai leads to the formation of the 

 archenteric cavity. This groove is formed by delamination ; until 

 now there is no trace of invagination. This begins in what Brachet 

 calls the second phase of the gastrulation process, which leads to the 

 formation of the archenteric cavity in its entire width, and is 

 synchronic with the process of notogenesis, of the formation of the 

 back of the embryo; "quand les lèvres blastoporales se soulcvent, 

 quand de virtuelles elles deviennent réelles, c'est que le blastopore 

 va com meneer a se fermer, c'est que le dos de l'embryon va 

 commencer a se former" (I.e. 1902, p. 225). 



Brachet is right here. Also there, where be draws a sharp line 

 between the entirely embryogenic blastoporus of the holoblastic eggs 

 and the blastoporus of the meroblastic eggs with a large amount of 

 yolk, which is divided into two parts, an embryogenic blastoporus 

 and a yolk-blastoporus. 



But when he reckons these processes, which occur in the selachian 

 and teleostean egg during the covering of the large mass of yolk 

 and the closure of the blastopore, still to gastrulation, when he calls 

 the entire process of covering of the yolk "clivage gastruléen'', and 

 calls the whole blastoderm ring "blastopore virtue]", he goes too far, 

 and forgets the significance of the phenomena, occurring at the end 

 of segmentation and during the formation of the periblast. 



For the answer to the question, at what time does the gastru- 

 lation in the teleostean egg begin, his analysis of the phenomena 

 of this process in the amphibian egg is extremely interesting. 



The segmentation of the teleostean eggs is not regular during all 

 its phases. When we combine the very accurate observations of 

 Kopsch on this account, we see, that in the segmenting blastoderm 

 at a definite moment, about that of the lO 1 ' 1 division of the embryonic 

 cells, there occurs an important alteration. 



Until the end of the 10 th cell-division (in Belone) the different 

 cells divide wholly synchronic; in Torpedo Rückert found synchro- 

 nism until the 9 th division. By the tenth division the yolk-sac ento- 

 blast is formed (in Gobius, Crenilabrus, Belone), the two nuclei of 

 the marginal segments, resulting from this division, remaining in the 

 undivided protoplasm; where this does not occur at the tenth division 

 the deviation is very small (in Cristiceps argentatus it partly begins 

 at the 9 th division, in Trutta fario at the eleventh division). Syn- 

 chronically with the differentiation of yolk-sac entoblast the super- 

 ficial layer ("Deckschicht") is differentiated. At the end of the 10 th 

 division all at once the blastoderm alters its form : it gets higher, 

 more hill-shaped and the diameter is lessened; the mass of cells 



