CHANGES BETWEEN 40 AND 50 HOURS 77 



until the original orientation of the chick on the yolk is com- 

 pletely changed. As the body of the embryo becomes turned 

 on its side the yolk no longer impedes the progress of flexion. 

 Following the accomplishment of torsion in the cephaHc region, 

 the cranial flexure becomes rapidly greater until the head is 

 practically doubled on itself (Fig. 34). As development pro- 

 ceeds, torsion progresses caudad involving more and more of 

 the body of the embryo. Finally the entire embryo comes to 

 lie with its left side on the yolk. Concomitant with the progress 

 of torsion, flexion also appears farther caudally, affecting in 

 turn the cervical, dorsal, and caudal regions. The series of # 

 flexions which accompany torsion bend the head and tail of 

 the embryo ventrally so that its spinal axis becomes C-shaped 

 (Fig. 40). The flexions which bend the embryo on itself so 

 the head and tail lie close together are characteristic of all 

 amniote embryos. The torsion which in the chick accompanies 

 flexion is correlated with the fact that it develops on the surface 

 of a large yolk. 



The Completion of the Vitelline Circulatory Channels. — In 

 chicks of 3^_to36hours the omphalomesenteric veins have been 

 established as postero-lateral extensions of the same endocardial 

 tubes which are involved in the formation of the heart. As 

 the omphalomesenteric veins are extending laterad, the vessels 

 developing in the vitelHne plexus are extending and converging 

 toward the embryo. Eventually the vitelline vessels attain 

 communication with the heart by becoming confluent with the 

 omphalomesenteric veins. This establishes the afferent chan- 

 nels of the vitelline circulation. 



The vessels destined to carry blood from the embryo to the 

 vitelline plexus develop in embryos of about j -ohours (Fig. 29). 

 Like the afferent vitelline channels, the efferent channels have " 

 a dual origin. The proximal portions of the efferent channels 

 arise within the embryo as branches of the dorsal aortae, and 

 extend peripherally. The distal portions of the channels arise 

 in the extra-embryonic vascular area and extend toward the 

 embryo. The efferent vitelline vessels are estabhshed when 

 these two sets of channels become confluent. In its early stages 

 the connection is through a network of small channels rather 

 than definite vessels, the aortae breaking up posteriorly into 

 small channels some of which communicate laterally with the 



