ORIGIN OF DEFINITIVE SEX-CELLS 449 



ORIGIX AND :\IIGRATIOX OF THE PRIMORDIAL GERM-CELLS 



Before commencing a description of the evolution of the gonad 

 and its tissues, it seems best to enter into a short account of the 

 early history of the primordial germ-cells, which are present in 

 the germinal region before the appearance of the gonad, and 

 which enter into it at its inception. 



The primordial germ-cells of the chick, according to Swift 

 ('14), arise from germ-wall entoderm anterior and antero-lateral 

 to the forming embryo. This genesis occurs during the primi- 

 tive streak stage and continues until the embryo possesses about 

 3 pairs of somites. 



This region of the young embryo is not invaded by mesoderm 

 until later so that the germ-cells are first seen free in the space 

 between ectoderm and entoderm. They later, on account of 

 their amoeboid properties, enter the mesoderm, and a great 

 majority of them get into the forming blood channels. They are 

 carried in the blood vessels to all parts of the embryo and vascular 

 area. The primordial germ-cells remain generally distributed 

 in this way until the embryo has about 20 pairs of somites. 



Dantschakoff ('08) described certain cells in the chick embryo, 

 which resemble in every way the primordial germ-cells during 

 their intra-vascular life. She called them entodermal wander- 

 cells, and stated that they disappeared from the blood vessels 

 when the embryo had about 22 pairs of somites. 



The primordial germ-cells are still found in the blood vessels 

 of embryos having from 20 to 22 pairs of somites, but are becom- 

 ing more numerous in the vessels of the splanchnic mesoderm, 

 while in embryos possessing 23 to 25 pairs of somites a majority 

 of them are found in the mesodermal plate of the splanchno- 

 pleure near the coelomic angle. 



Many investigators, among whom may be named Nussbaum 

 ('80, '01), Kiebel and Abraham ('00), Hoffmann ('93), Rubasch- 

 kin ('07), and von Berenberg-Gossler ('12), have described the 

 germ-cells in the splanchnopleure of bird embryos having 22 to 

 23 pairs of segments. At this age, as will have been seen from 

 the foregoing, they have just passed out of the vessels. 



