128 THE PHYSIOLOGY OF REPRODUCTION 



Immediately after copulation, and therefore during cestrus, 

 the cells of the discus proUgerus (in the rabbit) ^ begin to with- 

 draw radially, and eventually remain attached to the zona 

 radiata by the extremely thin strands just referred to. At the 

 same time the ovum itself withdraws somewhat from the zona, 

 leaving a narrow circular space. These processes occupy some 



Fig. 31. — Human ovum at termination of growth period. (After van 

 der Stricht.) Yolk granules, vacuoles, and tat drops are seen. 



hours. About nine hours after copulation, when the supply of 

 nourishment has been entirely cut off, the two polar bodies are 

 formed, and the ovum becomes mature. 



The essential facts about the maturation process were first 

 ascertained by van Beneden ^ in Ascaris, and were afterwards 



' Heape, " Ovulation and Degeneration of Ova in the Rabbit," Proc. Roy. 

 Soc, B., vol. l.xxvi., 1905. 



2 Van Beneden, "Kecherches sur la Maturation de TOSuf," Arch, de Biol., 

 vol. iv., 1883. 



