124 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



ing into the dorsal portion of the cloaca ; and then this cloacal 

 region becomes constricted off from the rest to form a urinary 

 bladder, which is connected directly or indirectly with the ex- 

 terior by a single duct, the urethra. The bladder persists 

 throughout life in lizards, turtles, and mammals, but disappears 

 in the other amniotes. 



Reproductive Organs. To those structures which are to 

 produce the reproductive cells, eggs and spermatozoa, the 

 term gonads has been given. These are paired (unless fusion or 



pe 



IG. 132. Section of ovary of new-born child, from Hertwig after Waldeyer. 

 /, single egg surrounded by follicle cells ; g, group of egg cells and follicle cells ; 

 ge, germinal epithelium ; p, egg strings ; pe, primordial ova ; z>, blood-vessels. 



abortion of one occur), and arise from the epithelium lining 

 the body cavity, nearer the median line than the nephrosto- 

 mata (see Fig. 127, go). In this region, which may extend 

 nearly the length of the body cavity or which may be more 

 restricted, the epithelium retains its original columnar character, 

 and even increases it, while in all other regions it becomes 

 converted into a pavement epithelium. The underlying mesen- 

 chyme increases in amount, pushing the germinal epithelium out 

 into the body cavity as a longitudinal ridge. It is usually 

 stated that in the earlier stages the gonads are segmental in 



