AVES. 265 



The circulatory organs are not complicated by the presence of 

 gills. The heart at first consists of two longitudinal vessels that 

 fuse to form a simple tube, which first becomes twisted and 

 then chambered. From its front end aortic arches run up the 

 visceral arches (Fig. 80), and unite above to form the dorsal 

 aorta. 



1st and 2nd Arches become (in part) . carotids. 



3rd Arches ,, subclavians. 



(right, . . aorta. 



4th Arches '\ . abo rts. 



5th Arches ,, ... pulmonary arteries. 



The veins of the embryo at first (3rd day) consist of paired 

 anterior cardinals ( = adult jugulars) and posterior cardinals, which 

 bring back blood from the anterior and posterior parts respec- 

 tively (cf. Dogfish, p. 165). The anterior and posterior cardinal 

 of each side unite to form a Cuvierian vein ( = adult precaval), 

 which enters the heart. The posterior cardinals are supplanted 

 later on by the development of a postcaval. 



The course of the embryonic circulation will be discussed 

 below. 



Excretory Organs. A very rudimentary pronephros is first 

 developed, with a pronephric duct (= archinephric or segmental 

 duct) which runs back and opens into the cloaca. It is succeeded 

 by a mesonephros (Wolffian body), the duct of the pronephros 

 becoming the mesonephric (Wolffian) duct. The mesonephros does 

 riot, however, become the excretory organ of the adult, but is 

 succeeded by a metanephros, or true kidney, for which a special 

 metanephric duct (ureter) is developed. The only conspicuous 

 remnant of the mesonephros that persists after embryonic life is 

 its duct, which becomes the spermiduct of the male. 



4. Embryonic Appendages (Fig. 82). These are important 

 structures connected with, but not forming part of, the embryo. 

 To understand them, it is not only necessary to remember that 

 the blastoderm gradually grows round the yolk, but also that the 

 mesoderm splits here, as elsewhere, so that continuations of soma- 

 topleure, splanchnopleure, and ccelom are present. As the embryo 

 is folded off from the yolk, this, with the covering of splanchno- 

 pleure, comes to form a sort of bag, the yolk-sac, attached to the 

 ventral side of the embryo by a stalk continuous with the wall of 

 the gut. The yolk is gradually used up during development, so 



