III.] 



THE NEUKENTERIC PASSAGE. 



73 



into the notochord, -where the latter joins the primitive streak 

 (B). A narrow diverticulum from this passage is continued for- 

 wards for a short distance along the axis of the notochord (A, 

 cK). After traversing the notochord, the passage is continued 

 into a hypoblastic diverticulum, which opens ventrally into the 

 future lumen of the ahmentary tract (0). Shortly behind the 

 point where the neurenteric passage communicates with the 

 neural tube the latter structure opens dorsally, and a groove on 

 the surface of the primitive streak is continued backwards from 

 it for a short distance (0). The first part of this passage to 

 appear is the hypoblastic diverticulum above mentioned. 



Fig. 26. 



Diagrammatic Longitudinal Section through the Pos- 

 terior End of an Embryo Bird at the time op the 

 Formation of the Allantois. 



ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric 

 canal ; hy. hypoblast ; p.a.g. post-anal gut ; pr. remains of 

 primitive streak folded in on the ventral side ; al. allantois ; 

 me. mesoblast ; an. point where anus will be formed ; p.c. 

 perivisceral cavity ; avi. amnion ; so. somatopleure ; sp. 

 splanohnopleure. 



In the chick we have found in some cases an incomplete pas- 

 sage prior to the formation of the first somite. At a later stage 



