514 CHARLES H. SWIFT 



SUMMARY AND CONXXUSIONS 



1. The primordial germ-eells arise anterior and anterc-lateral 

 to the embryo in a specialized region of germ- wall entoderm just 

 at the margin of the area pellucida. This region has roughly 

 the shape of a crescent and the germ-cells arise during the primi- 

 tive streak stage and until the embryo has about 3 somites. 

 The concavity of this crescent is towards the embryo and the 

 horns extend caudalward on either side. 



2. Owing to the late appearance of mesoderm in this region, 

 the primordial germ-cells at first are in the space between ento- 

 derm and ectoderm. Later, thanks to their amoeboid power, 

 they enter the mesoderm and the forming blood vessels of the 

 mesoderm. 



3. They are at first carried by their own movement, and later 

 by that of the blood to all parts of the embryo and vascular area. 

 They remain generally distributed in this way until the embryo 

 has about 20 somites. 



4. In embryos with about 20 to 22 pairs of somites, the primor- 

 dial germ-cells, while generally distributed throughout the em- 

 bryo in the blood-vessels, are becoming relatively more numerous 

 in the vessels of the splanchnic mesoderm. This increase in the 

 number of the germ-cells in the vessels of the splanchic mesoderm 

 may be in part only apparent, that is, a degeneration of some 

 may have occurred elsewhere, or, it may be a real increase due 

 to some influence, probably of a chemotactic nature, exerted in 

 the region of the future gonad. At this period the great major- 

 ity of the cells are found in the vessels, but a few, chiefly in the 

 splanchnic mesoderm, are present in the tissues. In some cases 

 they are present in the wall of the vessel, as if fixed in the act 

 of leaving the vessel for the tissues (fig. 11). 



5. In embryos with about 23 to 25 pairs of somites the major- 

 ity of the primordial germ-cells are found in the mesodermal tis- 

 sue of the splanchnic mesoderm near the angle of the coelom. 

 A few cells are still present in the blood-vessels ; at the 25th somite 

 stage, in one embryo, in addition to the few found in the vessels 

 of the splanchnic mesoderm, four masses of germ-cells are in the 



