xii. 22 REPRODUCTION 341 



(to be distinguished from the mesodermal bladder of fishes) from 

 which water can be reabsorbed. Certain desert amphibia (Chiroleptes) 

 conserve water by losing the glomeruli altogether. Rana cancrivora is 

 euryhaline and may have 2-9% of urea in the blood (Gordon 1961). 



The Miillerian duct, by which eggs are carried to the exterior, 

 develops separately from the Wolffian system in the frog, but arises 

 from the latter during development in urodeles. In this, as in many 

 other features, the frog shows a greater degree of specialization of its 

 developmental processes. The ovaries are mere folds of the peri- 

 toneum, having no solid stroma such as is found in mammals. There 

 are, however, follicle cells around each egg; these presumably pro- 

 duce the ovarian hormones. Sections of an ovary show eggs in various 

 stages of development, but not all those that begin complete their 

 maturation; many degenerating, atretic eggs are found. Ripening of 

 the eggs proceeds under the influence of a hormone produced by the 

 anterior lobe of the pituitary. This in turn is controlled by external 

 environmental factors to ensure breeding in the spring. Suitable 

 injections of mammalian anterior pituitary extracts will ensure ripen- 

 ing of the ovaries and ovulation at any time of year. The 'prolans' 

 excreted in the urine of pregnant women have a similar effect, and 

 the production of ovulation in Xenopas is used as a test for the diagno- 

 sis of human pregnancy. 



Having left the ovary the eggs find their way to the mouths of the 

 oviducts mainly by ciliary action of the latter. The walls of the oviduct 

 are glandular and secrete the albumen; they are dilated at the lower 

 end to form uterine sacs, in which the eggs are stored until laid. 



The testes discharge directly through the mesonephros by special 

 ducts, the vasa efferentia, formed by outgrowths from the mesone- 

 phros into the gonad. This is presumably a secondary development 

 from the original vertebrate condition in which the sperms were 

 carried away by the nephrostomes. The fact that the sperms pass 

 through the kidney emphasizes that the amphibia have diverged at 

 a very early stage of the evolution of the vertebrate stock, and remain 

 still in many respects at a lower level of evolution than the modern 

 fishes, all of which have acquired separate urinary and genital ducts. 

 In Alytes, in many ways primitive, the sperms do not, however, pass 

 through the kidney! 



In some frogs (R. temporaria) there is a special diverticulum, the 

 vesicula seminalis, leading by several small channels to the lower end 

 of the Wolffian duct. It contains spermatozoa during the breeding- 

 season and its appearance suggests a secretory activity. 



