TJROGENITAL SYSTEM. 319 



free life is entirely absorbed or is lost with the placenta. In the 

 amphibia the urine finds its way into the urinary bladder via the cloaca, 

 as the urinary ducts (Wolffian ducts) do not open into it. In those 

 amniotes in which a bladder is present the ureters open into it, and 

 the urine is conveyed to the exterior by a single tube, the urethra. 

 In many sauropsida there is no urinary bladder, though the allantois 

 is formed in development. 



There is great difficulty in comparing the excretory system of the vertebrates 

 with anything known in the invertebrates. In general the nephridial tubules may % 

 be compared with those of the annelids. Both have nephrostomes opening into 

 the ccelom, 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 (p 312) 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 myotomes 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 mesoderm the homology falls to the ground and an explanation is still a 

 desideratum. 



THE REPRODUCTIVE ORGANS. 



The tissue which is to form the ovaries and testes early forms a 

 pair of genital ridges, one on either side of the mesentery and between 

 it and the Wolffian ridge (fig. 319). At one time it was thought that 

 the anlage of the gonad was segmental in character and 'gonotomes,' 

 comparable to nephrotomes and myotomes, were described. It has 

 since been shown that no metamerism exists and that the primary 

 germ cells, which alone characterize the gonads, arise in several groups 

 of vertebrates (possibly in all) from the entoderm, which is never 

 metameric. At about the time of the differentiation of the somites 

 they migrate through the developing mesoderm to their definitive posi- 

 tion in the epithelium of the genital ridges, the primitive or primordial 

 ova (whether to form eggs or sperm) being recognizable from their 

 size and their reaction to microscopic stains (fig. 324, 0). In the 

 adults of many vertebrates the gonads at maturity project far into 

 the ccelom and are often suspended by a fold of peritoneum which is 

 called a mesorchium in the male, a mesoarium in the female. 



