V.] THE MESENTERY. 127 



actual termination of the tail, and certain features connected 

 with the development of the allantois, of which we shall 

 speak presently, the tail is a counterpart of the head. . 



So rapid is the closure of the splanchnopleure both in 

 front and behind, that two of the three parts into which 

 the digestive tract may be divided, are brought, on this day, 

 to the condition of complete tubes. 



The first division, extending from the mouth to the 

 duodenum, is completely folded in by the end of the day ; so 

 likewise is the third division comprising the large intestine 

 and the cloaca. The middle division, corresponding to the 

 future small intestine, still remains quite open to the yolk- 

 sac below. 



The attachment of the newly formed alimentary canal to 

 the body above is at first very broad, and only a thin stratum 

 of mesoblast separates the hypoblast of the canal from the 

 notochord and protovertebrse ; even that may be absent under 

 the notochord (Fig. 41). During the third day, however, 

 along such portions of the canal as have become regularly 

 enclosed, i. e. the hinder division and in the posterior moiety 

 of the anterior division, the mesoblastic attachment becomes 

 narrower and (in a vertical direction) longer, the canal appear- 

 ing to be drawn more downwards (or according to the position 

 of the embryo forwards), away from the vertebral column. 



In what may be regarded as the pleural division of the 

 general pleuroperitoneal space, along that part of the ali- 

 mentary canal which will form the oesophagus, this with- 

 drawal is very slight (compare Fig. 32), but it is very marked 

 in the peritoneal space. Here such parts of the digestive 

 canal as are formed come to be suspended from the body 

 above by a narrow flattened band of mesoblastic tissue which 

 reaches from the neighbourhood of the notochord, and be- 

 comes continuous with the mesoblastic coating which wraps 

 round the hypoblast of the canal. This flattened band is the 

 mesentery, shewn commencing in Fig. 44, and much more 

 advanced in Fig. 47, M. It is covered on either side by a 

 layer of flat cells, while its interior is composed of indifferent 

 tissue. 



The front division of the digestive tract consists of three 

 parts. The most anterior part, the oesophagus, still ends 

 blindly in front, and reaches back as far as the level of the 



