506 



CHARLES H. SWIFT 



Add to this similarity the facts (1) that, in embryos younger 

 than 21 somites these large characteristics cells are all in the ves- 

 sels; (2) that in embryos with 21 and 25 somites respecti\'ely they 

 are in the tissues as well as in the vessels (figs. 4, 6, 7, 8 and 10) ; 

 and (3) that, in embiyos older than this they are found in the 

 tissues of the splanchnic mesoderm alone; so that there can be 

 no question as to the fact that the large, characteristic cells in 

 the blood vessels are primordial germ-cells and are identical with 

 the germ-cells found in the splanchnic mesoderm. 



Fig. 6 Portion of a transverse section through the head region of a 21 somite 

 embryo. Aleves' fixation and Meves' iron-hematoxylin stain. X 787. -pr.o., 

 primordial germ-cell near fore-brain; pr'.o'., primordial germ-cell in a blood vessel 

 near optic cup; vii., yolk sphere; /.5., fore-brain; o.c, optic cup; ep., epidermis. 



In the embr^'o with 21 mesoblastic somites there are present 

 altogether 73 primordial germ-cells. Of these, 13 are outside the 

 blood-vessels, nearly all in the splanchnic mesoderm, and 60 are 

 contained within vessels throughout the embryos and area vas- 

 culosa. Of these 60, in the vessels of the splanchnic mesoderm, 

 there are 30, while in vessels of the head region 25 are to be seen 

 (fig. 6). The distribution of the germ-cells in the splanchnic 

 mesoderm is of interest in that all except two or three are caudad 



