150 BERTRAM G. SMITH 



free cytoplasm, and underlying this a thicker layer of especially 

 fine yolk particles rich in cytoplasm. Both layers are continuous 

 with much thinner layers of the same character surrounding the 

 remainder of the egg. 



In the ovocyte ready for maturation, the germinal vesicle lies 

 close to the surface at the animal pole, and is surrounded by the 

 germinal disc. A mass of cytoplasm has accumulated beneath 

 the germinal vesicle during the later stages of its migration. The 

 arrangement of materials is now quite strongly telolecithal. 



Shortly before the rupture of its wall, the germinal vesicle 

 appears at the very surface at the animal pole. The rupture of the 

 germinal vesicle takes place just before the egg leaves the ovary; 

 the cytoplasmic and yolk layers of the blastodisc now mingle, 

 and the materials of the germinal vesicle, together with the cyto- 

 plasm brought with it from the interior of the egg, are incorpor- 

 ated into the blastodisc. 



Absorption of degenerating ovocytes is accomplished by means 

 of the follicle cells, which reverse their usual role as nurse cells 

 of the egg, and function as phagocytes. 



The first polar spindle is formed about the time the egg leaves 

 the ovary, and disappears about the time the egg enters the uterus. 

 There are marked size differences in the chromosomes. 



The second polar spindle is formed shortly after the egg enters 

 the uterus; it lies beneath a deep pit readily visible from the 

 surface. 



The penetration of the egg by the spermatozoon is not required 

 as a stimulus to the formation of the second polar spindle. 



The late stages of the second maturation division, culminating 

 in the formation of the second polar body and the egg -nucleus, are 

 passed through only after the spermatozoon has entered the egg; 

 in other words, the processes of maturation and fertilization 

 overlap. 



A structure resembling a micropyle is formed in the cell wall of 

 the egg around 'the perforation made by the entrance of the sper- 

 matozoon. The influence of the entering spermatozoon upon the 

 egg is shown by characteristic changes in the distribution of the 

 yolk and cytoplasm. 



