596 GEORGE T. HARGITT 



the mass of the gonad which protrudes into the sub-umbrellar 

 cavity. These proliferating ectodermal cells are the oogonia 

 or spermatogonia of the gonads, for it is clearly shown in sec- 

 tions that there is practically no difference between the germ 

 cells and the outer covering of the gonad. Indeed it is possible 

 in almost any section of young gonads to see the ectodermal 

 tissue of the outside of the gonad, and the mass of germ cells 

 more or less intermingling. The cells of the radial canal and 

 the cells of the entodermal lining of the cavity of the gonad are 

 distinctly a gastrovascular tissue. They are long columns, with 

 the cytoplasm much vacuolated, the nuclei are smaller and have 

 an appearance different from those of the ectoderm and the 

 germ cells. It is by a continued proliferation of the originally 

 few germ cells that the large gonad filled with germ cells is 

 produced. 



OOGONIA 



The proliferation of the original germ cells is rapid and there 

 is produced an enormous mass of these cells. The divisions are 

 mitotic in character, apparently without exception, and the pro- 

 liferating period is more or less limited and regular, since a 

 cessation of division takes place in all cells of the gonad about 

 the same time. The ending of the division period is not abrupt, 

 for a few mitoses may be found scattered through the oogonia 

 after the process has apparently come to an end, but in a full- 

 sized gonad filled with oogonia it is only by long search that a 

 few divisions can be found. After growth has begun in some 

 of the cells of the ovary there has never been found a case of 

 dividing oogonia. The early stages in oogenesis are, therefore, 

 quite regular and uniform; such variations as occur in older 

 gonads are due to differences in the time at which the growth 

 period began, with perhaps some possible variations in the rate 

 of growth. 



Figure 1 represents a portion of one side of an ovary and shows 

 the long columnar epithelial cells of the gastro-vascular cavity, 

 the mass of germ cells, and the ectodermal covering of the 

 gonad. In this particular ovary oogonial divisions had ceased 



