470 



HISTOLOGY 



FIG. 440. Oogonia from ovary of 

 kitten. Multiplying by mitotic 

 divisions. X 1000. 



When first seen in this embryonic material the reproductive cells 

 are oogonia of exceedingly small size, smaller than the epithelial cells, 

 and they appear to originate from the epi- 

 thelium or else directly under it, probably 

 the former. They form the groups which 

 we have called the tubules, even before 

 they have left the surface. In other mam- 

 mals they have been described as carrying 

 an invaginated tube of the epithelium into 

 the ovary with them. They certainly do 

 not do this in the cat, and when they have 

 passed in, clear of the basement mem- 

 brane, it can be seen that they are sur- 

 rounded by connective tissue. These ova- 

 rian tubules have been compared to the 

 seminiferous tubules of the male. 



The nucleus of each cell is very round, and the nucleolus lies near the 

 middle, with a reticulum of strands of linin and chromatin reaching out 

 from it to the nuclear membrane. Cells nearest the center of the ovary 

 in each group are somewhat the largest, and the groups inside of the 

 first are larger than these outer ones. The figure (439, B) shows such a 

 larger group which has moved into the ovary a trifle farther inside than 

 the first. 



These cells undergo a few oogonial divisions and rapidly increase in 

 size as they move inward (Fig. 440). The nucleus increases at a greater 

 proportional rate, and opens up its chromatin pattern. A denser area 

 of the cytoplasm then ap- 

 pears in one side of the cell, 

 and the nucleus contracts 

 its chromatin into the syn- 

 izesis stage. Figure 441 

 shows a part of the cells 

 from a tubule whose cells 

 are either in synizesis or 

 preparing to perform it. It 

 can be seen that a black 

 central granule is found in 

 the dense area of cytoplasm, 

 and, in consequence, the 

 structure has often been de- 

 scribed as a centrosome. By 

 other writers it is called a yolk nucleus, and considered homologous 

 with the yolk nucleus of spiders, fishes, and other mammals. It per- 



FlG. 441. Oogonia of kitten undergoing synizesis. 

 Yolk body present in some. X 1000. 



