III.] THE NEURENTERIC PASSAGE. 71 



process the primitive streak undergoes important 

 changes both in itself and in its relation to the embryo. 

 Its anterior thicker part, which is embraced by the 

 diverging medullary folds, soon becomes distinguished 

 in structure from the posterior part, and is placed 

 symmetrically in relation to the axis of the embryo, 

 (Fig. 23 a.pr); at the same time the medullary folds, 

 which at first simply diverge on each side of the 

 primitive streak, bend in again and meet behind so 

 as completely to enclose this front part of the primi- 

 tive streak. The region, where the medullary folds 

 diverge, is known as the sinus rhomboidalis of the 

 embryo bird, though it has no connection with the 

 similarly named structure in the adult. 



This is a convenient place to notice remarkable appearances 

 which present themselves close to the junction of the neural plate 

 and the primitive streak. These are temj)orary passages leading 

 from the hinder end of the neural groove or tube into the alimen- 

 tary canal. They vary somewhat in different species of birds, and 

 it is possible that in some species there may be several openings 

 of the kind, which appear one after the other and then close again. 

 They were first discovered by Gasser, and are spoken of as the 

 J^3ieure7iteric passages or canals^. In all cases, with some doubtful 

 ' exceptions, they lead round the posterior end of the notochord, or 

 through the point where the notochord falls into the primitive 

 streak. 



The largest of these passages is present in the embryo duck 

 with twenty-six mesoblastic somites, and is represented in the 

 series of sections (Fig. 25). The passage leads obhquely back- 

 wards and ventralwards from the hind end of the neural tube 



1 "Die Primitivstreifen bei Vogelembryonen." Schrift. d. Gesell. 

 z. Beford d. Gesammten Naturwiss. zu Marhurg. Vol. ii. Supple- 

 ment I. 1879. 



