138 EMBRYOLOGY. 



existed here from the beginning between the inner and the outer 

 germ-layers ; further, that this communication has disappeared 

 through the fusion of the blastoporic hps, but that it can be in part 

 reestablished in consequence of more favorable processes of growth. 

 At the same time the neurenteric canal, in cases where it reappears 

 in the primitive streak, effects a very characteristic union between 

 the posterior ends of the neural and intestinal tubes, in exactly the 

 same manner in which the blastopore of Amphioxus, the Amphibia, 

 and the Selachii does (compare fig. 80 with fig. 88 n.e). 



In the interpretation of the primitive groove as blastopore I am 

 compelled to oppose a somewhat different view. Certain investi- 

 gators (Balfour, Rauber, and others) recognise in the primitive 

 groove and the crescentic groove of meroblastic eggs only a small 

 part of the blastopore ; they interpret as the major part of it the 

 region which is encircled by the whole rim of the germ-disc and is 

 occupied by the yolk-mass, and to which they give the name yolk- 

 blastopore.* According to their conception, as also according to 

 the original assumption of Haeckel, the two-layered germ-disc is a 

 flattened-out gastrula, — its blastoporic rim lying upon the yolk- 

 sphere, — which gradually grows around the yolk, and finally takes 

 the latter wholly inside itself, just as if it were a ball of food. The 

 primitive groove is a small detached part of the blastopore, which is 

 connected with the development of the middle germ -layer. The two 

 parts become completely separated from each other, and are closed 

 at different times, each for itself, the yolk-blastopore often late, at 

 the pole of the yolk-sac which is opposite to the embryo. 



Such an assumption of a double blastopore appears to me to be 

 untenable. I propose tJiat only that place of the germ he designated as 

 blastopore at which, as in the gastruldtion of Amphioxus cmd the 

 Amphibia, there actually occurs an invagination of cells, by means of 

 which the cleavage-cavity is obliterated. Such a process takes place 

 in the Selachii only at the crescentic hinder part of the margin of 

 the germ-disc, in the Reptiles and Birds at the small place designated 

 as crescentic groove. It is also from this place alone that subse- 

 quently the development of the middle germ-layer proceeds. 



The anterior margin of the germ-disc in Selachians, and, after the 

 conversion oj the crescentic groove into the primitive groove, the whole 



* Eaubbe has suggested for the various regions which he assumes for the. 

 blastopore the designations prostoma sulcatum longitudinale (primitive groove), 

 prostoma milcatwm falalforme (ci'escentic groove), and jjrostoma marginale 

 <yolk-blastopore). 



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