FEMALE REPRODUCTIVE CELLS 



469 



one of close apposition, and the chromosomes of each, 18 in number, 

 remain independent of each other for a long time after, forming the 

 36 chromosomes of the regular starfish cells. They each divide in sub- 

 sequent divisions, so that every descendant has 36 chromosomes, and 

 each of these 36 was derived from one, and a different one, of the ori- 

 ginal 36 in the newly fertilized ovum or oosperm. Later, when some 

 of the descendants have become the male or female reproductive cells 

 of the young animal, and they are beginning to mature, they will at last 

 unite the paternal and maternal chromosomes into 18 bivalent chromo- 

 somes, which are the tetrads of maturation. This process is called 

 synapsis, and sometimes occurs during synizesis or contraction. It 

 has been seen in a few cases, in some Hemiptera, where it is described 

 by Wilson, McClung, and others. 



A study of the mammal ovum will yield a fuller account of its very 

 early development than that of the starfish does, and therefore will 

 serve as a concrete example of those first stages. Like the male re- 

 productive cells of the skate, 

 the female cells of the mammal 

 may first be seen in the outer 

 part of the germinal ridges, two 

 longitudinal thickenings of the 

 embryonic body-cavity wall of 

 the body cavity. Later, these 

 folds become developed into the 

 ovaries, two separate bodies of 

 mesodermal cells, each covered 

 with a mesothelium. The re- 

 productive cells appear as pri- 

 mordial egg cells, in or just un- 

 der the mesothelium, which is a 

 part of the peritoneum reflected 

 over the ovary. 



At a very early date (before birth in many mammals), all the repro- 

 ductive cells which are going to pursue a further development are drawn 

 down in groups into the body of the ovary. These groups we shall term 

 egg tubules in general. Figure 439, A, shows such a group, or tubule, 

 just below the germinal epithelium in the ovary of an embryo of the cat, 

 in which we can most easily trace the early stages in the history of the mam- 

 mal ovum. These cells are young oogonia, and they move inward from 

 the epithelium, developing as they pass toward the inner part of the ovary. 

 This movement is continued until there is a thick, cortical layer of young 

 ova a short distance under the epithelium and extending about a third 

 of the diameter of the ovary inward. 



B 



FIG. 439. Two young egg tubules near the sur- 

 face of the ovary in a kitten, g.e., germinal epi- 

 thelium. A, youngest egg tubule. B, older 

 tubule. X 1000. 



