84 THE ANATOMY OF VERTEBRATED ANIMALS. 



msceral cleft is left unnumbered, and one must be added to the number of each bran- 

 chial cleft to give its number in the series of visceral clefts. 



B. Hypothetical diagram of the aortic arches in the Shark HeptancJiUS, which has seven 



branchial clefts. Sj). The remains of the first \'isceral cleft as the sphacle. Branchise 

 are developed on all the arches. 



C. Lepido&iren. — The first arch has disappeared as such, and the first visceral cleft is ob- 



literated. Internal branchiae are developed in connection -with the second, filth, sixth, 

 and seventh aortic arches; external branchias in connection with the fourth, fifth, and 

 sixth. PA. — The pulmonary artery. The posterior two visceral clefts are obliterated. 



D. A Teleosteau Fish. — The first aortic arch and fu-st visceral cleft are obhterated as before. 



The second aortic arch bears the pseudo-branchia (Ps.-S.), whence issues the ophthalmic 

 artery, to terminate in the choroid gland {Ch.). The next four arches bear giUs. The 

 seventh and eighth arches have been observed in the embryo, but not the ninth, and 

 the included clefts are absent in the adult, 



E. The Axolotl {Siredon), a perennibranchiate amphibian. The third, fourth, fifth, and 



sLxth aortic arches, and the anterior four branchial clefts, persist. The first visceral 

 cleft is obliterated. 

 E The Frog. — The three anterior aortic arches are obhterated in the adult. The place of 

 the third, which is connected with the anterior external gill in the Tadpole, is occupied 

 by the common carotid and the rete mirabile (carotid gland, Ca. G.) which terminates 

 it. The fourth pair of aortic arches persist. The fifth and sixth pair lose their connec- 

 'tions with the subvertebral aortic trunk, and become the roots of the cutaneous and 

 pulmonary arteries. The first visceral cleft becomes the tympanum, but all the others 

 are obhterated in the adult. 



The embryonic aorta gives off omphcdomeseraic branches 

 (Fig. 26, 6) to the umbilical vesicle ; and ends, at first, in the 

 hypogastric arteries (which are distributed to the allantois in 

 the abranchiate Vertehrata)., and a median caudal continuation. 

 The blood from the umbilical vesicle is brought back, as before 

 mentioned, by the omphalomeseraic veins (Fig. 26, o'), which 

 unite in a dilatation close to the head ; the dilatation (smus 

 ve?iosus) receives, on each side, a short transverse venous 

 trunk, the ductus Cuvieri (Fig. 26, DC)., w^hich is itself 

 formed, upon each side, by the junction of the anterior and 

 posterior cardinal veins, which run backward and forward, 

 parallel with the spine, and bring back the blood of the head 

 and of the trunk. 



The blood of the allantois is returned by the umbilical 

 vein., or veins (Fig. 26, i«'), which are formed in the anterior 

 wall of the abdomen, and open into the venous sinus before 

 mentioned. The blood of the posterior extremities and kid- 

 neys is, after a while, brought to the same point by a special 

 median vein, the veiia cava inferior (Fig. 26, cv). 



The development of the liver effects the first great change 

 in the arrangements now described. It, as it were, interrupts 

 the course of the omphalomeseraic vein, w^hich is not only the 

 vein of the umbilical sac but also that of the intestine, and 

 converts it into a mesh work of canals, which communicate, on 

 one side, w4th the cardiac part of the vein, and, on the other 

 Bide, with its intestinal part. The latter is thus converted into 

 the ve7ia portce (Fig. 26, vp)., distributing the blood of the 

 stomach and intestines to the liver ; w^hile the former becomes 



