THE OAT. 



[CHAP. 



subsequent development, the part of the loop pre-axial to this point 

 of attachment, increases in length vastly more than does the part 

 post-axial to it. The part in front (pre-axial) becomes the duode- 

 num, jejunum, and the greater part of the ileum. The part behind 



f / 



Fig. 155. LONGITUDINAL VERTICAL SECTION THROUGH AN EMBRYO, SHOWING THE INCIPIENT 

 INTESTINE WITH THE RELATIONS TO IT OF THE LUNGS AND LIVER. 



d. Intestine. 



0. Mouth. 

 a. Anus. 



1. Lungs. 

 7i. Liver. 



g. Mesentery. 

 v. Auricle of heart. 

 Ic. Ventricle of heart. 

 &. Arterial arches. 

 L Aorta. 



c. Yelk-sac. 



m. Duct leading from the intestine to the yelk- 

 sac. 



. Allantois. 

 r. Stalk of allantois. 

 11. Amnion. 

 v). Cavity of amnion. 

 s. Outer layer of primitive amniotic fold, which 



is separating off to coalesce with the inside 



of the primitive chorion. 



(post- axial to) the attachment of the umbilical vesicle, becomes the 

 rest of the ileum and the colon. Thus that proximity of the transverse 

 colon to the stomach and duodenum which we have seen in the 

 adult condition, is initiated at first ; the long intervening tract 

 being formed by the outgrowth and coiling of the primitive loop, the 

 distal ends of which (stomach and transverse colon) preserve very 

 nearly their primitive relations. The distinction between the large 

 and small intestine is first indicated by a small prominence, the 

 future ccecum, which appears a little post-axial to the attachment of 

 the vitelline duct of the umbilical vesicle. The large intestine is, at 

 first, of less capacity than the small. 



The formation of what are ultimately the two ends of the 

 alimentary canal, namely the mouth and anus, has already been 

 incidentally referred to. The primitive pre-axial end of the alimen- 

 tary canal reaches beneath the skull, where it ends blindly below 

 the pituitary space and forms the oesophagus and pharynx. 



The anterior end of the canal is at first closed by the lamina 

 yentralis, which descends in front of it as a single plate, not divided 

 into an outer somatopleure and an inner splanchnopleure. In the 



