DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



137 



medullary furrow, with its small circular opening filled up with a 

 yolk-plug. 



In addition there are three important considerations which may 

 be urged in support of the interpretation of the primitive groove as 

 blastopore. 



First, the primitive streak, even when an open canal is wanting, 

 is the only place in the whole germ-disc where a connection between 



Fig. 101. A and B . a. portion of a yovnTer and of an older embryonic fundament of Emya 



Europsea, with the prostoma or blastopors (), after KUPFFER. 

 ill, Lip of the blastopore. 

 C and D. Two eggs of Triton tseniatus seen from the blastopore, one 30 hours, the other 53 hours 



after artificial fertilisation. 

 , Blastopore ; h, elevation between blastopore and dorsal groove ; /, semicircular furrow, which 



encloses the blastoporic area ; dp, yolk-plug. 



Jill the germ-layers is constantly present, as at the Amphibian 

 blastopore. 



Secondly, the chief organs of the body, such as the chorda, the 

 neural tube, and the primitive segments, are developed in front of 

 the primitive streak in the case of the higher Vertebrates, just as 

 they arise in front of the blastopore in Amphioxus and the Amphibia. 

 Both blastopore and primitive streak occupy the posterior end of the 

 body. The so-called cephalic process of the primitive streak is 

 nothing else than the first rudiment of the chorda. 



Thirdly, one may still recognise in the openings canales neu- 

 renterici which have been pointed out in the primitive streak at an 

 earlier or later stage in its development, in the case of Birds, Reptiles, 

 and Mammals, an indication that an open communication has 



