568 



VERTEBRATE LIFE AND ORGANIZATION 



Fertiliza-tion 



Ovula-tion 



FedlopicLn toLc 



Figure 28.6. A diagram to show the path of an egg from the ovary to the uterus, 

 and the clianges that occur en route. The last stage is about a week and one half old. 

 (Modified after Dickinson.) 



Other times, they are withdrawn into the abdominal cavity. Spermato- 

 genesis, like other vital processes, can only occur within a limited 

 temperature range. Apparently this range is exceeded by the tempera- 

 ture in the abdominal cavity, but not by the temperature in the scro- 

 tum, which is approximately 4° C. lower. In order to test this hypoth- 

 esis. Dr. Carl R. Moore of the University of Chicago confined the testes 

 of rats to the abdominal cavity and found that spermatogenesis did 

 not occur. Indeed, the seminiferous tubules underwent regression. He 

 also insulated the scrotum of a ram in which the testes were descended. 

 This raised the temperature, and again spermatogenesis did not oc- 

 cur. Apparently during the evolution of homoiothermism in mammals 

 spermatogenesis did not become adapted to the higher body temperatures. 



241. Reproductive Passages 



Once the sperm and eggs have been produced, they must be re- 

 moved from the body and be brought together to form a zygote. This 

 is a simple procedure in primitive vertebrates such as cyclostomes. No 

 reproductive ducts are present, and both eggs and sperm simply break 

 out of the gonad into the coelom. Ciliary currents carry them to the 

 posterior end of the coelom where they are discharged through a pore 

 into the cloaca. Fertilization and development are external. 



Embryonic Formation of Reproductive Ducts. Other vertebrates 

 have a system of ducts for the removal of the gametes, and some of 

 them are intimately related to the excretory system. In order to under- 

 stand this relationship, it is necessary to go back to a period in em- 



