FEMALE REPRODUCTIVE CELLS 



461 



the most interesting and difficult subject in cytology. Although its 

 essential features have been already described in the reduction of male 

 reproductive cells, we 

 shall outline it again as 

 seen in the female cells, 

 following this outline 

 by several concrete de- 

 scriptions, and finally 

 by some account of the 

 union of the male and 

 female gametes to form 

 the oosperm or zygote. 



The young female 

 reproductive cell, before 

 it begins its growth, is 

 known as an oogonium. 

 It possesses the same 

 number of chromatic 

 units or chromosomes that are characteristic of the somatic cells of the 

 species. The beginning of maturation (which is a comparatively rapid 

 process occurring during the breeding season) is marked by a gathering 

 of the chromatin into a closely reticular mass that lies at one side of the 

 nucleus. This process was formerly known as synapsis but, as synapsis 

 is now used as a term to designate another process which may take place 

 during this closely reticular stage or before it (usually during the telophase 

 of the last oogonial division), the term synizesis has been used to desig- 

 nate the close reticulum. 



Upon emerging from synizesis, the oogonium begins a quick period 

 of growth or yolk accumulation, by the means already described, and 



FIG. 426. Young ovum of the angler, Lophius piscatorius. 

 Shows the yolk body beneath the nucleus, x 500. 



FIG. 427. Three stages in the growth period of the egg of Pholcus. The yolk body appears 

 as a ring about the nucleus, and swells and disintegrates as the yolk accumulates. (From 

 WILSON after VAN BAMBEKE.) 



then becomes an oocyte of the first order. It now forms its chromosomes 

 for the two divisions known as the reduction divisions. If the somatic 

 number of chromosomes during a division is 12, we shall find that the 



