550 CUORDATA. 



-mals. Keptiles and amphibia show how the modification has been 

 accomplished. In these the separation begins in the venous sys- 

 tem and extends to the auricle, in the reptiles the septum arises in 

 the ventricle. In the arterial system remnants may persist, such 

 as a connexion (ductus Botalli) of the pulmonalis with the aorta 

 (//, d.B), or an aortic arch may arise with the pulmonalis from 

 the right side of the heart (III, ao). 



Besides blood-vessels, lymph vessels occur in the vertebrates as com- 

 plements of the venous system. The fluids which collect in the spaces of 

 the connective tissue are taken by them and carried into the large venous 

 trunks. Usually the action of the heart and the movements of the body 

 are sufficient to cause the flow of this lymph, but special lymph hearts 

 may occur. The lymph vessels distributed to the digestive tract play an 

 important role, since they serve in the resorbtion of digested food. They 

 are called chyle ducts because their contents, the chyle, rendered white 

 by oil globules at the time of digestion, distinguishes them from other 

 lymphatics. The most important features of lymph and blood have 

 already been noticed (p. 88). In special places small bodies, the lymph 

 glands, are inserted in the course of the lymph vessels, in which lymph 

 corpuscles arise. Among these from its structure is to be enumerated the 

 spleen, colored bright red by its rich blood supply. 



The sexual and excretory organs are so closely associated that 

 they are generally united as the urogenital system. The sexual 

 products are formed in the embryo from a special region of the 

 peritoneal epithelium on either side of the vertebral column. 

 These primordial cells early leave their primitive position, and sink 

 into the underlying connective tissue (fig. 33), forming in the 

 male glandular tubes, in the female cords which break up into 

 numbers of round follicles, each containing a single larger cell, the 

 ovum. In the male the gonads thus formed are compact and fre- 

 quentlv oval, the testes; in the female they are looser and follic- 

 ular ovaries. 



The deposition of the sexual cells occurs in many fishes by way 

 of the body cavity and the abdominal pores, and in this case a part 

 of the ccelom may be cut off as a special vas deferens or oviduct. 

 In most vertebrates the ducts are formed from a part of the 

 nephridial system. Embryology shows that there are three kinds 

 of nephridia in vertebrates: (1) the pronephros, or head kidney; 

 (2) mesonephros, or Wolffian body; (3) metanephros, or kidney 

 proper, with the corresponding pronephric, mesonephric (Wolf- 

 fian), and metanephric (uretei) ducts. The first two of these 

 ducts are genetically connected, since the development of the 

 elasmobranchs shows that the pronephric duct, by splitting, gives 



