52 Jenkinson, Germinal Layers of Vertebrates. 



into position by the bilateral closure of a blastopore and 

 simultaneous formation of an archenteron. These 

 processes originate here, as in the Birds, exclusively in 

 the upper layer, though subsequently a secondary 

 fusion with the paraderm occurs. The blastopore 

 may be laterally compressed — a primitive groove — and 

 the archenteric cavity practically absent or reduced to 

 a very narrow ' neurenteric ' or 'chorda-canal'; but in 

 some cases — the best instance is Vespertilio (van Beneden) 

 {^Fig. 31, A) — the blastopore and archenteron are as well 

 developed as in the Reptiles. The archenteron always 

 comes into secondary communication with the subgerminal 

 blastocystic cavity. In front of the dorsal lip the noto- 

 chord is formed in the ordinary manner from the roof of 

 the archenteron or mass of cells — ' Kopffortsatz ' of the 

 older authors — which is its representative ; from the sides 

 of the blastopore spring the lateral sheets of mesoderm 

 confluent, as in other cases, with one another behind 

 {^Fig. 31, B). According to many authors, the blastoporic 

 lips — the primitive groove — are the sole seat of formation 

 of notochord and mesoderm, but Heape has described in 

 Talpa, and Hubrecht in Sorex and Tar sins, the formation 

 of an anterior portion of both notochord and mesoderm 

 from the paraderm ; '" and the latter author has further 

 observed the origin of a peripheral ring of mesoderm 

 from the lower layer in the same two genera. This 

 peripheral mesoderm has been recorded by Bonnet in the 

 sheep, where, however, it is denied by Keibel. 



The paraderm — growing in from the sides — seems to 

 complete the endodcrm in all cases. 



^" Mr. Assheton informs me that this also occurs in the Rabbit. 



