322 Papers from the Department of Marine Biology. 



The germ-cells are apparently most abundant at about the 11-day 

 stage. This increase is probably merely apparent, due to the segrega- 

 tion of the cells within relatively more restricted and conspicuous 

 limits, viz, gut and mesentery. During earlier stages (to the seventh 

 day), when the gut is still entirely open over the yolk, their distribution 

 is scattered over a wider area ; also in later stages, when they have taken 

 their definite position within the enlarged gonad. The number of 

 germ-cells in the 11-day embryo, all situated behind the level of the 

 cephalic tip of the pronephric duct, is approximately 400. Only an 

 occasional cell may be seen at this stage in mitosis. Similarly, during 

 the later stages mitosis is a very rare occurrence. In the stages of 

 2 to 4 days a few of the germ-cells are in process of division among 

 the entodermal cells. Cells in division are apparently of smaller size 

 and contain relatively little yolk. 



10-DAY EMBRYO; HELLY FIXATION, GIEMSA STAIN. 



In a 10-day embryo the primordial germ-cells are found in about 

 the same locations; they are slightly smaller, but the smaller size is 

 only apparent, due to a greater cytoplasmic shrinkage in this fluid 

 (fig. 4a, plate 1), in part also very probably to the loss of the large 

 yolk-content. Moreover, the Flemming fluid may have caused a 

 slight swelling. The cytoplasm is here vacuolated and coarsely retic- 

 ular, the result of the solution of yolk-granules. In figure 4, a, 6, c, 

 d, and e, plate 1, are shown primordial germ-cells from the gut, mesen- 

 tery, and peritoneal epithelium of this stage. For comparison a 

 blood-granulocyte (/) is added. 



12-DAY EMBRYO; HELLY FIXATION, IRON-HEMATOXYLIN STAIN. 



At the twelfth day the germ-cells are still scattered among the ento- 

 dermal cells of the hind-gut, in the mesentery, and in the genital ridge. 

 Figure 6a, plate 1 , is taken from among the entodermal cells of the gut ; 

 6, from among the mesothelial cells of the genital ridge; c, from the 

 mesenchyma of the mesentery. The adjacent mesenchymal cells 

 have become changed by the pressure of the migrating cell into flat- 

 tened encapsulating elements. 



The accompanying chart shows the distribution of the germ-cells 

 at this stage, and their condition in different locations. The total 

 number of cells counted is 352, about equally divided between the 

 two gonads (118 in left, 127 in right). A few cells are in mitosis, 

 mostly situated among the entodermal cells. Some are in process of 

 degeneration; such are found chiefly in the mesenchyma of the gut. 

 One cell is situated in what appears to be a vascular space, close to 

 the aorta; another is in process of degeneration within a blood-vessel; 

 seven occur in the periaortic mesenchyma ; and four among the neuro- 

 blasts of the developing periaortic sympathetic ganglia (fig. 2, plate 4). 1 



lr The photomicrographs were made by Mr. Wm. S. Dunn, Cornell University Medical School 

 New York. 



