CHAP. XIII 



CIRCULATORY ORGANS 



599 



ventral side of the pharynx. 

 It has at first the form of a 

 straight tube, developed in 

 the chick and rabbit by the 

 fusion of two longitudinal 

 vessels (vitelline veins, see 

 below, and Figs. 161 and 164) ; 

 it soon becomes twisted into 

 an S-shape, and transverse 

 constrictions are formed 

 dividing it into the different 

 chambers. The auricular 

 and ventricular portions are 

 each at first single, but from 

 the Amphibia onwards the 

 former subsequently be- 

 comes divided into two by 

 a septum, and in birds and 

 mammals the ventricle is 

 similarly subdivided. The 

 modification of the arterial 

 arches in the examples 

 studied has already been 

 described (pp. 452 and 



523)- 



In the dogfish, bird, and 

 rabbit the dorsal aorta, in ad- 

 dition to its other branches, 

 gives rise to paired vitelline 

 arteries : these branch over 

 the extra-embryonic part of 

 the blastoderm (p. 585) 

 which spreads over the yolk- 

 sac, and in the two former 

 takes an important share 



bm 



vb-L 



vb 



ns 



FIG. 160. A section through a Chick 

 embryo at the end of the third day of 

 incubation, the section being taken 

 along a plane indicated by the two 

 arrows and crosses in Fig. 158, B ; the 

 right side of the section is at a level 

 slightly dorsal to that of the left 

 side. (X 30.) (After Marshall.) 

 a. dorsal aorta ; a 1 , first aortic arch, 

 in the mandibular arch ; a'-*, second 

 aortic arch, in the hyoid arch ; a 3 , 

 third aortic arch, in the first branchial 

 arch ; ac. carotid artery ; at. internal 

 carotid artery ; bm. cavity of mid- 

 brain ; 4rl. first branchial arch ; 

 cH. notochord ; km. hyomandibular 

 cleft ; hy. hyoid arch ; mn. mandibular 

 arch ; mp. muscle-plate ; ne. ganglion 

 of spinal nerve ; ns. spinal cord ; pt. 

 pituitary pouch ; tp. pharynx ; vb. 

 jugular vein. 



types the capillary network 

 in the absorption of the 



