172 THE THIRD DAY. [CHAP. 



superficial epiblast except at the upper surface (corre- 

 sponding to the back of the embryo), where lies the 

 pointed termination of the neural tube. 



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, including the region from the 

 mouth to the duodenum, is completely folded in by the 

 end of the day; so likewise is the third division com- 

 prising 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 mesoblastic somites; 

 even that maybe absent under the notochord. During the 

 third day, however, along such portions of the canal as 

 have become regularly enclosed, i.e. the hinder division 

 and the posterior moiety of the anterior division, the 

 mesoblastic attachment becomes narrower and (in a ver- 

 tical direction) longer, the canal appearing to be drawn 

 more ventralwards (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 alimentary canal which will form the oesophagus, 

 this withdrawal is very slight (Fig. 59), 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 mesoblas- 



