ORIGIN OP PRIMORDIAL GERM CELLS 497 



ning with these older forms, one can become famihar with the 

 germ-cells in regions where they have been seen and studied be- 

 fore, and acquire a more perfect technique, before following them 

 back through an unknown migration to an equally unkno\^m and 

 obscure origin. 



In an embryo of four and one-half days incubation the germi- 

 nal epithelium is found to be developed along the medial surface 

 of the Wolffian body. It is composed of cells which differ from 

 those of the general coelomic epithelium in being cuboidal or 

 columnar. Owing to the greater height of these cells a ridge- 

 like appearance is produced on the medial surface of the Wolffian 

 body, which may simulate a developing gonad. The gonad itself 

 is forming only at the level of the anterior half of the Wolffian 

 body, where stroma is beginning to appear beneath the germinal 

 epithelium. The developing mesentery, springing from between 

 the Wolffian bodies, has attained a length easily appreciated by 

 the unaided eye. 



Scattered here and there in the stroma of the developing gonad, 

 in the root of the mesentery, both in the mesenchymal tissue and 

 in the coelomic epithelium near the coelomic angle, and in the 

 germinal epithelium, clothing the gonad, are certain cells which 

 are sharply marked off from the neighboring cells. They are the 

 primordial germ-cells which have previously been described in 

 this article, and which have been noticed and studied at this 

 stage and this region by all investigators of germ-cell history in 

 the chick since the publication of Waldeyer's "Eierstock und Ei." 



By far the greater number of the primordial germ-cells in the 

 four and one-half day embryo are found in the gonad, and in the 

 root of the mesentery near the coelomic angle, but some are to 

 be seen in other situations, namely, in the mesenter>^ at a dis- 

 tance from its point of attachment to the body, in the wall of 

 the gut, in the mesenchyme behind the aorta and in front of the 

 notochord and even an occasional one dorsal and lateral to the 

 notochord. 



The germinal epithelium in the chick of four and one-half days 

 of incubation is well developed. It consists of a strip of cuboidal 

 to columnar cells, without any basement membrane, which on 



