EXISTI XG ORGANISMS— EMBRYOLOGY 



65 



In the embryos of birds and mammals the heart is at first a 

 simple tube, as in such simple chordates as Amphioxus. The first 

 dill'crcntiation of this tube is its separation into four regions 

 similar to those found in the hearts of adult fishes, viz., the; sinus 

 venosus, atrium, ventricle and Imlbus arteriosus. Later the first 

 and last are absorlwd into the adjacent regions, the auricle is 

 divided by a partition, and finally the ventricle is similarly divided, 



post, cardinal 

 vein 



coclom 



intermediate 

 mesoderm 



coelom 



aephrostoxn& 



glomus 



notochord 



somite 



dorsal aorta 



post, cardinal 

 vein 



ncphrostome 



coelom 



somite 



dorsal aorta 



capsule 



D 



mesonepbric 

 tubuie 



mesonepbric 

 duct 



ncphrostome 

 ' gfomerulus 



Fig. 44. — The structure of nephric tubules. A, pronephric tubule from a 

 16-somite chick embryo; B, diagram of functional pronephric tubule; C, 

 primitive mesonephric tubule with rudimentary nephrostome, from a 

 3()-somite chick embryo; D, diagram of functional mesonephric tubule of 

 the primitive type. (P'rom Patten, A after Lillie, B and D after Wieder- 

 sheim; with the permission of P. Blakiston's Son & Co.) 



SO that the organ goes through a transition approximating that 

 represented by the several classes (Fig. 42). 



While the heart is developing, a series of aortic arches appear, 

 six pairs in all, which are at first symmetrical and unlike those of 

 the fish chiefly in the lack of capillaries (Fig. 29). As the ventricle 

 divides, the truncus arteriosus splits so that the separation of that 

 portion leading into the last pair of arches, and thus to the lungs, 



