XII.] THE MESENTERY. 419 



enlarged to form the large intestine, while the anterior 

 portion becoming very much elongated and coiled forms 

 the small intestine, and moreover gives rise anteriorly 

 to the liver and pancreas. 



From the large intestine close to its junction with the small 

 intestine an outgrowth is developed, the proximal part of which 

 enlarges to form the ccecum, while the distal portion in Man 

 forms the vermiform appendix. 



3. The postanal division of the mesenteron atro- 

 phies at an early period of embryonic life. In the Chick 

 and lower types it communicates for a short time with 

 the hind end of the neural canal. 



Splanchnic mesoblast and mesentery. The mesen- 

 teron consists at first of a simple hypoblastic tube, which 

 however becomes enveloped by a layer of splanchnic 

 mesoblast. This layer, which is not at first continued 

 over the dorsal side of the mesenteron, gradually grows 

 in, and interposes itself between the hypoblast of the 

 mesenteron, and the organs above. At the same time 

 it becomes differentiated into two layers, viz. an outer 

 epithelioid layer which gives rise to part of the peritoneal 

 epithelium, and an inner layer of undifferentiated cells 

 which in time becomes converted into the connective 

 tissue and muscular walls of the mesenteron. The 

 connective tissue layers are first formed, while of the 

 muscular layers the circular is the first to make its 

 appearance. 



Coincidently with the differentiation of these layers 

 the connective tissue stratum of the peritoneum becomes 

 established. 



The mesentery is developed as in the Chick (p. 172). 

 In the thoracic region it is hardly if at all developed. 



272 



