Embryonic History of the Germ-Cells of the Loggerhead Turtle. 319 



also are the granules of the confusing large blood granulocytes. The 

 germ-cells in the gonads of the 25-day embryo are practically iden- 

 tical with those of the 11-day embryo in form and size and in regard 

 to cytoplasmic content. Comparative study of the 11-day embryo 

 fixed and stained in the three different ways — (1) Flemming's fluid and 

 iron-hematoxylin stain; (2) Helly and iron-hematoxyhn ; (3) Helly 

 and Giemsa — revealed admirably the diagnostic marks between the 

 germ-cells and cells with which they might be confused, i. e., blood 

 granulocytes, yolk-laden hemoblasts, and yolk-laden entodermal cells. 

 These marks once established at this stage, earlier stages could be 

 much more readily identified in the Helly-Giemsa or Helly-iron-hema- 

 toxyhn material than in the Flemming material, since the most con- 

 fusing and obscuring features were here largely eliminated, namely, 

 the very abundant yolk globules and granules. 



DESCRIPTIVE. 

 11-DAY EMBRYO; FLEMMING FIXATION. IRON-HEMATOXYLIN STAIN. 



In view of what was said above it seems clear that the best way of 

 approach to the problem was through the 11-day embryo preserved in 

 Flemming's fluid and stained with iron-hematoxyhn. The next step 

 involved a comparative study of embryos of this same stage fixed with 

 Helly's fluid and stained respectively with the iron-hematoxyhn and 

 the Giemsa mixtures. Thus the criteria for identifying the primordial 

 germ-cell were established and the previous and later history could 

 then be traced with comparative ease. 



This stage corresponds closely with von Berenberg-Gossler's illus- 

 tration (fig. 9) of a Uzard embryo. Primordial germ-cells are present 

 among the entoderm-cells of the closed hind-gut and in the surrounding 

 mesenchyma; in the mesentery, both in the mesenchyma and its 

 mesothelial covering; and in the primitive genital gland, both among 

 the peritoneal cells and in the subjacent mesenchyma (see fig. 2, 

 plate 6). Figure 1, plate 1, is a composite drawing including the 

 several types of cells with which we are now concerned, all from the 

 same immediate region of the mesentery. We may compare first cells 

 a and h, a typical germ-cell and a typical eosinophil granulocyte, both 

 in the mesenchyma of the mesentery. Cell-tjrpe h may appear at any 

 stage after the second day of incubation in any part of the yolk-sac 

 or embryonal mesenchyma, including the blood-vessels. At first sight 

 it would seem to correspond to what Swift described as a primordial 

 germ-cell circulating in the blood-vessels of the young chick embryo. 

 It contains an attraction sphere (i), a spherical deep-staining chro- 

 matic nucleus, and abundant small spherical granules (apparently 

 comparable to the granular mitochondria of germ-cells); but the re- 

 semblance is only very superficial and no confusion need arise. 



