UROGENITAL SYSTEM 811 



nephridial tubules may be compared with those of the annelids. Both 

 have nephrostomes opening into the coelom, and convoluted tubules 

 enveloped in a network of capillary blood-vessels, but in the annelid each 

 tubule opens separately to the exterior in the somite behind that in which 

 the nephrostome lies, while in the vertebrate the series of tubules empty 

 into a common duct. When it was thought that the ectoderm con- 

 tributed to the pronephric duct, the homologies appeared easy. The 

 duct was originally a groove on the outer surface into which the separate 

 tubules opened. Then the groove was rolled into a tube which continued 

 backward to the vicinity of the anus. By the downgrowth of the myo- 

 tomes the duct became cut off from its primitive position and came to lie 

 just outside the peritoneal lining. When, however, it is considered that 

 in all probability the pronephric duct is formed solely from the meso- 

 derm the homology falls to the ground and an explanation is still a 

 desideratum." 



THE REPRODUCTIVE ORGANS. 



A detailed study of the embryological beginnings and development 

 of the reproductive organs has already been covered in that part of this 

 book devoted to embryology. After this has been reviewed it will be 

 understood how the germ plasm is early set aside in the growing embryo 

 from which the gonads develop . The gonads are not segmented notwith- 

 standing the fact that earlier writers have taken another view. 



These sexual organs in their growth, push a layer of peritoneum be- 

 fore them just as do the other outgrowths in the body. Such peritoneum 

 covering the male gonad, which serves as a support for the testes is 

 called a mesorchium, while that supporting the ovaries is known as a 

 mesovarium. In all the higher forms gonads are paired. In many fishes 

 and birds they are unpaired, but this is due to a fusion of two or degen- 

 eration on one side. 



We have seen in our embryological study how the eggs are formed 

 in the female and lie within a Graafian follicle which, after rupture, leaves 

 a scar in the form of a corpus luteurn, while in the male, instead of the 

 primordial ova and the epithelial cells becoming separate follicles they 

 develop into a cord which later on has a lumen open through it and be- 

 comes the seminiferous tubule. Both epithelial cells and primordial 

 spermatagonia may be found in the walls of this tubule. A third type, 

 known as Sertoli's cell, is also found here. These latter are called nutri- 

 tive or nurse cells for the developing spermatozoa. Just what function, 

 aside from this supposed nursing, these cells have is unknown. The 

 testes remain in the position where they first appear in most vertebrates, 

 while in nearly all the mammals they descend, assuming a position out- 

 side the body cavity, being enclosed in a special pouch called the 

 scrotum. 



THE REPRODUCTIVE DUCTS. 



As fertilization is necessary in at least all the higher forms of 

 animals, there must be some method by which the sperm or the eggs 



