336 OUTLINES OF CHORD ATE DEVELOPMENT 



entiated into the anterior proventriculus, with thick glandular 

 walls, and the posterior gizzard, with its thick muscular coats. 

 The horny lining of the gizzard is derived from the secretion of 

 specific glands in its own wall. 



Beginning the third or fourth day of incubation the mid-gut, 

 including the stomach, commences to elongate, and as a result 

 the tract becomes first simply looped, and then complexly 

 folded (Fig. 132). First the stomach bends to the right, return- 

 ing to the median region near its opening into the intestine. 

 The first section of the intestine is the duodenum; this is a very 

 short section, receiving the ducts of the liver and pancreas. 

 The duodenum elongates very little and remains as a relatively 

 fixed region, closely attached to the dorsal body wall. Simi- 

 larly the terminal portion of the intestine, the rectum and large 

 intestine, elongates only slightly. Between the duodenum 

 and the large intestine the jejunum or vitelline portion of the 

 small intestine elongates considerably, and is consequently 

 thrown first into an S-shape. The yolk-sac connects with the 

 apex of the lower loop (Fig. 132). Later this whole section 

 shows secondary loops or twistings along each primary loop. 

 At the connection of the small and large intestines the two 

 intestinal cceca grow out during the second week. 



IV. THE VASCULAR SYSTEM 



The formation of the vascular system and its development up 

 to the thirtieth hour, we have already described; and we have 

 also mentioned the important steps in the development of the 

 extra-embryonic circulation (yolk-sac and allantois). We 

 must now trace the important steps in the development of the 

 true embryonic circulation, from the stage where we left it. 

 Let us recall that we left the heart as a bent tube, suspended in 

 the pericardial cavity; anteriorly it leads, by way of the first 

 or mandibular pair of aortic arches, into the dorsal aorta, the 

 chief branches of which (vitelline arteries) supply the extra- 

 embryonic organs. The embryonic venous system at that 

 time consisted only of the roots of the vitelline veins, returning 



