CHAPTER 10 



The immature egg and its maturation 



a. Growth and Differentiation of the Oocyte. Plate II 



The Arbacia egg matures in the ovary, so that when the eggs are shed 

 they have lost their polar bodies and are fully ripe and ready for fertil- 

 ization. The various stages of growth of the young oocyte and the polar 

 body formation are best studied at Woods Hole in early or middle 

 June. One can obtain immature eggs among the mature ones later in 

 the summer, but they are usually not abundant. 



The very young oocytes, of about 14 [j, in diameter, have a very large 

 germinal vesicle with a vesicular nucleolus, very little cytoplasm, 

 sometimes granular, sometimes quite clear, and no pigment. (Plate II, 

 Photograph i). From the data given in Table i, it will be seen that 

 at this stage the nuclear material (germinal vesicle) occupies about 

 two thirds of the whole cell and that there is more than twice as much 

 nuclear material as cytoplasmic. As the immature egg increases in 

 size, the germinal vesicle also increases, but not so much as the cyto- 

 plasm. The largest immature egg is the same size as the mature egg, 

 but the mature nucleus is very much smaller than the germinal vesicle 

 of the immature egg, about i/40th the volume. 



The young oocytes are still without pigment till they reach a dia- 

 meter of about 33 [i. (Photograph 3). They then become slightly pig- 

 mented, the pigment granules, gradually increasing in quantity with 

 growth. The oil, recognizable as an oil cap after centrifuging, appears 

 about the same time as the pigment. The mitochondria appear around 

 the germinal vesicle at about the same time, as can be determined by 

 staining with methyl green. A few scattered mitochondrial granules 

 are sometimes present in smaller eggs, of about 23 [jl in diameter. Jelly 

 is formed when the young oocyte is about 60 ix in diameter (Photo- 

 graph 6). The data are given in Table i. 



No study has been made of the oogenesis and spermatogensis of 

 Arbacia, but Tennent and Ito (1941) have published a very complete 

 account of the oogenesis of the Japanese form, Mespilia globulus, which 

 is probably similar. 



