CHAP, 



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 



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 - 



dC- 



-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 :{ . 

 third aortic arch, in the first branchial 

 arch ; ac. carotid artery ; at. internal 

 carotid artery ; bm. cavity of mid- 

 brain ; 6rl. first branchial arch ; 

 ch. notochord ; Am. hyomandibular 

 cleft ; hy. hyoid arch ; mn. mandibular 

 arch ; nip. 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 



