DEVELOPMENT OF THE TWO MIDDI.r. (-HUM I 



137 



m dtillary furrow. \\ it li it^ .small t-iirular op.-nii!i: lill.d up with n 



yolk-plug. 



In addition tin re are three important oonaderatkxu whiob : 

 he uri: d ill support of tin- interpretation of tin- primitive <:n><> 



pore. 



Kir.st, the primitive streak, even when an opm canal is \vair 

 is tlu- only ]>lace in the whole germ-disc where a connection 



Fig. 101. A and B. A portion of a younger and of an older embryonic fundament of Emys 



Europaea, with the prostoma or blastopore do, aft-r KITKFKK. 

 /, Lip of tho blaatopore. 

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



after artificial fertilisation. 



M, Bhistoiwre ; h, elevation between blastopore and dorsal groore ; /, semicircular furrow, which 

 a the blastoiwric area ; dp, yolk-plug. 



all the germ-la y. -r- is constantly present, as at the Amphibian 

 blastopore. 



Secondly, the chief organs of the body, MU!I as the chorda, the 

 neural tube, and the primitive segnu nt-. are drvrluprd in front of 

 the primitive streak in the case of the higher V. s, just as 



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

 Both blastoporo and primitive streak occupy the posterior end of the 

 body. The so-called cephalic process of the primitive >tnak 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 >tr ak at an 

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

 and Mammals, an indication that an open communication has 



