THE PROCTOD^EUM. 779 



with the exterior by means of an epiblastic invagination, consti- 

 tuting a proctodaeum. 



This invagination is not usually very deep, and in most 

 instances the boundary wall between it and the hypoblastic 

 cloaca is not perforated till considerably after the perforation of the 

 stomodaeum ; in Petromyzon, however, its perforation is effected 

 before the mouth and pharynx are placed in communication. 



The mode of formation of the proctodaeum, which is in 

 general extremely simple, is illustrated by fig. 420 an. 



In most forms the original boundary between the epiblast of 

 the proctodaeum and the hypoblast of the primitive cloaca 

 becomes obliterated after the two have become placed in free 

 communication. 



FIG. 429. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR 

 END OF AN EMBRYO BlRD 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. postanal 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. 



In Birds the formation of the proctodasum is somewhat more compli- 

 cated than in other types, owing to the outgrowth from it of the bursa 

 Fabricii. 



The proctodaeum first appears when the folding off of the tail end of 

 the embryo commences (fig. 429, an] and is placed near the front (originally 

 the apparent hind) end of the primitive streak. Its position marks out the 

 front border of the postanal section of the gut. 



The bursa Fabricii first appears on the seventh day (in the chick), as a 

 dorsal outgrowth of the proctodasum. The actual perforation of the sep- 

 tum between the proctodasum and the cloacal section of the alimentary tract 

 is not effected till about the fifteenth day of fcetal life, and the approxi- 



