1 68 



HISTORY OF THE GERMINAL LAYERS. 



a circular opening leading into the yolk-sack, which becomes 

 gradually narrowed as development proceeds. The opening is 

 completely closed long before the embryo is hatched. Certain 

 peculiarities in reference to the structure of the tail-fold are 

 caused by the formation of the allantois, and are described with 

 the embryonic appendages. The stomodaeum and proctodaeum 

 are formed by epiblastic invaginations. The communication 

 between the stomodaeum and the mesenteron is effected com- 

 paratively early (on the 4th day in the chick), while that 

 between the proctodaeum and mesenteron does not take place 

 till very late (i$th day in the chick). The proctodaeum gives 

 rise to the bursa Fabricii, as well as to the anus. Although the 



FIG. 106. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR 

 END OF AN EMBRYO BIRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. 

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

 blast; 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. peri- 

 visceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure. 



opening of the anus is so late in being formed, the proctodaeum 

 itself is very early apparent. Soon after the hinder part of the 

 primitive streak becomes tucked in on the ventral side of the 

 embryo, an invagination may be noticed where the tail of the 

 embryo is folded off. This gradually becomes deeper, and 

 finally comes into contact with the hypoblast at the front 

 (primitively the apparent hind) border of the posterior section of 

 the primitive streak. An early stage in the invagination is 

 shewn in the diagram (fig. 106, an}. It deserves to be noted 

 that the anus lies some way in front of the blind end of 



