6 



These vessels may be easily identified before the union of 

 the somatopleure to form the ventral wall of the embr3/o, and 

 at that period when the intestine forms a simple loop or bend. 

 This stage corresponds to an embryo of from six to ten 

 millimetres in length. 



Fig. 6 represents a frontal section of an 8-millimetre em- 

 bryo. The embryo was somewhat twisted, which made the 

 section a very fortunate one for my purpose, and the soma- 

 topleures have not as yet united. The loop of intestine is 

 easily recognized, with the longitudinally cut vitelline artery 

 in its mesentery. The artery crosses the intestine where the 

 loop is formed, and then passes on to the yolk sack, about 

 which it divides, and to which it is finally distributed. In a 

 more advanced stage of development, after the concrescence 

 of the somatopleure, the artery and vein are found in its 

 mesoderm previous to their division around the yolk sack. 

 This is well shown in Fig. 7, which represents a transverse 

 section of an embryo I 5| millimetres. In the cavity of the 

 umbilical cord a portion of the vitelline vein is seen, as well 

 as the artery and vein contained in the tissue of the soma- 

 topleure. 



As we have already said, the first step of the entrance of 

 the intestines into the abdominal ccelom is a simultaneous en- 

 trance of its two extremities, the caecal and jejunal. This 

 process continues until the progress of the ileum is arrested 

 by the vitelline artery, which crosses and fixes it, by means 

 of its attachment to the mesentery of the ileum on the one 

 hand, and to the tissue of the somatopleure and yolk sack on 

 the other. In other words, a portion of the ileum, situated a 

 few centimetres from the caecum, by means of these vessels 

 becomes anchored in the cavity of the umbilical cord. Its 

 further entrance is rendered impossible. 



Growth continues, and the jejunum, unhindered, enters the 

 ccelom proper, and then the ileum enters, until only a loop 

 of the ileum is left in the cavity of the cord, which is crossed 

 and fixed there by the vitelline vessels. 



These vessels now begin to elongate, and as they lengthen, 

 this coil of ileum is gradually allowed to enter the coelom, 



