Embryonic History of the Germ-Cells of the Loggerhead Turtle. 325 



gut ventrally. The germ-cells of the embryo, in contrast to all other 

 cells (except liver-cells) , still showed abundant yolk-granules and some 

 mitochondria. 



The migration period is obviously not sharply limited, but extends 

 in some degree through half the incubation period. Some germ-cells 

 apparently lag hopelessly behind and degenerate, especially in the gut. 

 The initial conspicuous marks of degeneration relate to stages in the 

 disappearance of the nucleus either by solution or karyorrhexis. 



YOUNGER EMBRYOS; FROM 9 TO 6 DAYS. 



The 9-day embryo (figs. 1 and 2, plate 5) differs from the 10-day 

 embryo only in the relative abundance of the germ-cells at different 

 levels of the migration route ; the germ-cells are relatively less abundant 

 in the higher levels and in the gonads. 



In figure 10a, plate 2, is illustrated a germ-cell of an 8-day embryo 

 taken from the splanchnic mesoderm at about the mid-region of the 

 area pellucida. It appears to lie in a lacuna, the product of shrinkage ; 

 6 is taken from the mesentery and is closely enveloped by mesenchymal 

 cells. The latter germ-cell has a diameter of only about 13 microns; 

 a, about 16 microns. For comparison, two hemoblasts are included, 

 c and d. The germ-cells at this stage are most abundant in the mesen- 

 tery and in the median portion of the splanchnopleure. 



In the 7-day embryo the hind-gut is open throughout that is, 

 beyond the anterior tip of the pronephros. The mesentery is relatively 

 very long caudally. Some germ-cells are found among the entodermal 

 cells medially, but the majority are in the long mesentery, widely 

 scattered, both among the mesenchymal and the mesothelial cells, and 

 in the medial extremity of the visceral plate of mesoderm. A few cells 

 are already found within the genital ridge and a few remain scattered 

 among the entodermal cells of the open gut. In figure 12, plate 2, 

 is illustrated a large germ-cell from among the entodermal cells of the 

 hind-gut, at the point of initial closure. The cytoplasm contains 

 enormous peripheral vacuoles and is much shrunken, due to the dis- 

 solution of the yolk-masses of the cell. Figure 11, plate 2, shows a 

 younger germ-cell among the entoderm-cells of the area pellucida, near 

 the mid-line. The difference in size of these two cells (16 microns and 

 20 microns) might be interpreted to indicate a considerable growth of 

 the germ-cells at about the 7-day stage, but it more probably indicates 

 merely a normal size variation, since large germ-cells appear already 

 at the 3-day stage (10 somites) (figs. 13, 14, and 15, plate 2). Swift 

 (1914) records a variation in diameter of the primordial germ-cells in 

 the chick embryo of from 14 to 22 microns, the average being about 16 

 microns. In this embryo a few germ-cells among the entodermal cells 

 of the open gut and in the mesothelium of the mesentery are in mitosis. 



