126 ANIMAL BIOLOGY. [Part I. 



and mesoblastic somites, its dorsal attachment is at first very 

 broad. As development proceeds the body-cavity extends 

 further and further round its sides and dorsal aspect until it is 

 only attached dorsally by a thin sheet of mesoblast. Meanwhile 

 it has been, so to speak, pulled downwards away from the 

 vertebral column, and has also increased in length to such an 

 extent as to be thrown into coils. The thin suspending sheet 

 of mesoblast has, during this process, been pulled downwards and 

 folded and thus converted into the mesentery. At the same 

 time, during the progress of these changes, the splanchnic layers 

 gradually close in so as to convert this region also into a tube. 



The posterior region of the mesenteron is at first in communi- 

 cation with a neurenteric canal similar to that of the frog (Fig. 

 36, n. e. c.). Ere long the communication of the neurenteric 

 canal with the medullary canal is obliterated. Meanwhile the 

 proctodaeum is being formed by special invagi nation from the 

 exterior to constitute the vent ; and the portion of the mesen- 

 teron and neurenteric canal posterior to the vent is known as 

 the post-anal gut. This part subsequently atrophies. The 

 greater part of the remainder of the posterior division of the 

 mesenteron forms the rectum. But its posterior portion that 

 which meets the proctodaeal invagination gives rise to that 

 inner part of the cloaca which receives the generative ducts, and 

 at this period of embryonic life, the allantois. 



The Proctodseum. The remainder of the cloaca, that which 

 constitutes its outer moiety into which the Bursa Fabricii opens, 

 is formed by the proctodseal invagination. This part is therefore 

 lined with epiblast, while the inner division of the cloaca is 

 lined with hypoblast. 



The Stomodseum and Nares. During the third day of in- 

 cubation two small depressions are formed at the anterior end of 

 the embryo. They are the nasal pits. During the next day the 

 mouth begins to be marked out as a deep stomodaeal depression, 

 somewhat behind and ventral to these pits. Fig. 40, A., shows 

 the changes which have taken place by about the sixth day. The 



