226 STRUCTURE OF THE VERTEBRATES 



B. Embryonic Circulation of the Amniotes 



The embryonic development of the vascular system closely 

 parallels the phylogenetic history. Therefore the embryonic cir- 

 culation of the higher groups gives a generalized idea of the adult 

 circulation of the lower forms. The following discussion is based 

 upon the embryology of the chick which is essentially like that 

 of the other amniotes, although the placentals have numerous 

 minor modifications in correlation with placental nutrition. The 

 student will recall that the reptile, bird and monotreme embryos 

 develop on large yolked eggs with meroblastic cleavage ; but that 

 the marsupials and placentals have small ova w^iich cleave 

 holoblastically. The latter develop a yolk sac upon which the 

 embryo grows in a typically amniote manner. 



In large-yolked eggs the outline of the embryo is laid out on 

 the blastodisc before the blood vessels begin development. The 

 first sign of the vascular system is a group of rounded blood 

 islands in the extra-embryonic mesoderm, each of which soon 

 begins to branch. As lumina (internal cavities) appear, the 

 branching arms fuse and the larger vessels begin to grow in 

 toward the embryo, until a series of vessels is formed on either 

 side of the embryo, AVithin the embryo a synchronous growth of 

 vessels has formed a pair of ventral veins which grow outward. 

 These unite with the extra-embryonic series of vessels and form 

 the vitelline veins, one on either side, which carry food materials 

 from the yolk to the embryo. 



A continued growth of the vitelline veins carries them an- 

 teriorly to the region of the pharynx, where they turn dorsally 

 and join the developing dorsal aortae. The dorsal aortae are at 

 first paired throughout their entire length, and continue poste- 

 riorly into the caudal end of the embryo. Immediately poste- 

 rior to the vitelline veins a group of capilliform vessels is given 

 from each aorta. These pass outward to the yolk and make 

 contact with the capillaries of the veins, thus completing the 

 earliest circulatory system. The small arteries to the yolk later 

 fuse to form a pair of large yolk sac arteries. At this time the 

 fundamental outline of the circulatory system has developed: (1) 

 a pair of incoming veins from the yolk to embryo; (2) an en- 

 larged region in the anterior portion of these vessels, destined 



