UROGENITAL 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 ceelom, 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 acommon duct, Whenit 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 coelom and are often suspended by a fold of peritoneum which is 
called a mesorchium in the male, a mesoarium in the female. 
