346 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



In the dipnoi there is a diverticulum from the dorsal wall of the 

 cloaca, anterior to the openings of the Wolffian ducts. This is usu- 

 ally called the urinary bladder (fig. 373 Z?), but it may be homologous 

 with the rectal gland of the elasmobranchs. 



The third type, the allantoic bladder, occurs in all tetrapoda. 

 This arises as a ventral diverticulum from the cloaca. In the am- 

 phibia the whole of the outgrowth forms the bladder and its walls 

 are supplied by the hypogastric arteries. In the amniotes the proxi- 

 mal portion alone is converted into the urinary bladder, while the 

 more distal portion, becomes in the embryo the respiratory organ of 

 the growing young, the allantois. This part extends far beyond the 

 body wall, carrying with it branches of the hypogastric arteries 

 (allantoic arteries), and in the mammals forms a part of the placenta. 

 The allantois becomes reduced in the later stages and at the beginning 

 of 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 directly 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 open- 

 ing 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 (p. 

 339) that the ectoderm contributed 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. 365). At one time it was thought that 



