SPERMATOGENESIS INSEMINATION 



165 



is transformed into the scrotum, which lies behind the penis (except 

 in Marsupials, where it is in front). In many Mammals the descent 

 takes place at an early age and is permanent. In others (most 



Spermatogonium. 



Oogonium. 



Proliferation 

 period. 



Growth period. 



Maturation 

 period. 



FIG. 51. Scheme of spermatogenesis and oogenesis. (After Boveri.) 



Kodents, Insectivores, and bats) the testes are withdrawn into the 

 abdominal cavity after the breeding season is over. This is effected 

 by the cremaster muscle. In Monotremes, Edentates, Cetacea, and 

 Proboscidea there is no descent. 1 



prepared by repeatedly injecting fowls with sperms of rabbits ; the sera are 

 stated to be toxic for sperms of rabbits and guinea-pigs when injected into the 

 blood at intervals for four or five weeks, pmrtial or complete sterility resulting, 

 the spermatogenetic tissue degenerating (" Studies on Cytolysins," III., Jour, of 

 Exp. Zool., vol. xxxv., 1922). Kuntz had found that after ligation of the vas 

 deferens in the rat the seminiferous tissue degenerated in both testes, and it is 

 suggested by Guyer that this result may have been due to spermatotoxins 

 resulting from resorption (Kuntz, Anat. ftec., vol. xvii., 1919). Kuntz's result is 

 not confirmed by other observers, and Guyer's suggestion seems very unlikely 

 on general grounds. 



1 Woodland has put forward a theory of the descent of the mammalian 

 testes involving the inheritance of acquired characters. His view is that the 

 descent has been caused by the action of mechanical strains resulting in rupture 

 of the attachments, such strains being due to the inertia of the organs (which 

 are larger than the ovaries of the female) reacting to the " impulsiveness " 

 (e.g. leaping or galloping movements) involved in the activity of the animals 

 (Proc. Zool. Soc., 1903). Cunningham (J. T.), who adopts this view, points out 

 that the descent takes place in the foetus and is related to the general habits 

 which begin soon after birth, and not to sexual habits (Hormones and Heredity, 

 London, 1921). 



