ORIGIN OF DEFINITIVE SEX-CELLS 461 



tance in the epithelium, nor are the cell outhnes distinct. The 

 layers can be distinguished more by the nuclei of the cells and 

 their relative position than by cell walls (fig. 6). However, 

 where a single cell can be seen in its entirety, it will be noted that 

 the form is columnar and the nucleus constituted as in the past 

 (compare figs. 1, 2 and 6). 



The region of the gonad under the germinal epithelium, which 

 may now be called the medullary region, is composed in greater 

 part of the medullary cords. They can be easily seen as cords 

 of epithelial cells, distinctly delimited, containing here and there 

 a primordial germ-cell, and at the same time showing evidences 

 of continued growth in the many mitotic figures. 



The gonad at this stage is increasing in all diameters. This 

 enlargement, as far as breadth and thickness is concerned, is 

 due in great measure to the increase in the medullary cords, 

 which fact is plainly evident on careful study of the transverse 

 section. 



The primordial germ-cells in the germinal epithelium are more 

 evenly distributed throughout this tissue than in any previous 

 stage (fig. 6). The greater number are still in that portion of the 

 epithelium near the mesentery but in an average transverse 

 section 6 to 12 are distributed in the rest of the germinal epithe- 

 lium over the main portion of the gonad. There are as yet no 

 evidences of division in the germ-cells but on the other hand the 

 vitellus is much reduced in amount. There also has been a 

 slight change in position of the rod-shaped mitochondria, which 

 are numerous in these cells. In former stages these bodies had 

 been rather evenly distributed in the cytoplasm, but now there 

 seems to be a tendency, rather slight it is true, for them to collect 

 in the region of the centrosphere. This is not true of all the 

 germ-cells, as is the case in older embryos; however, this fact 

 can be observed in a majority. 



Up to the stage now about to be described, that of 8 days, 

 there has been but little evidence of division of the primordial 

 germ-cells and most of that evidence has been of an indirect 

 nature — an increase in the actual number of germ-cells present 

 in each section from stage to stage, and their arrangement, at 

 times, in groups, as if indicative of past mitoses. 



