Frederick ildams Woods 309 



credits the view that these cells arise from epithelium and shoAvs that 

 the germ-cells are traceable out of the mesoderm as we seek them in 

 younger and younger embryos, until their origin is placed in the endo- 

 derm or underlying yolk, even before any mesoderm is formed. 



In fact, as far as determined, from their first appearance up to the 

 time of their situation in the genital region Avhen they become the 

 acknowledged primitive ova, these cells do not appear to be differen- 

 tiated at all. 



It is the other or somatic cells which become changed during the 

 processes of growth. 



In the youngest embryo studied, in the blastoderm stage, practically 

 all the cells, except those of the ectoderm have all the characteristics 

 of the primitive ova found in later embryos of from one to six milli- 

 meters. 



It will be seen from the drawings that while most of the cells of the 

 endoderm and mesoderm are losing their clear, round outline, and also 

 their yolk, fusing together into a net-work, or being changed into tall, 

 columnar epithelium, some cells retain all their original characteristics. 



It will also be seen that at least some, if not all of these cells of the 

 original type are gathered up in a small tumor-like ball at the junction 

 of the endoderm and mesoderm before the mesoderm has split, and that 

 afterwards these spread through this layer and by actual migration find 

 more and more their positions in the segmented mesoderm until they 

 finally rest exclusively in the genital region of this germ layer. During 

 no part of this migration have any cells been seen that would lead one 

 to suspect a transition of the mesoderm or mesothelial cells into primi- 

 tive ova. 



After the mesoderm is once formed and the ova are once in it, there 

 is no difficulty in determining whether any given cell is or is not a 

 germ-cell. 



Although several thousand were counted with oil immersion, no 

 doubtful cells were met. 



The ordinary cells of the mesoderm give no clear cell outlines, they 

 fuse together and are practically devoid of yolk. Such shapes as are 

 indicated are small spindles and cylinders. The primitive ova are large 

 oval or spherical, have clear cut cell walls, and are crowded with yolk. 

 A peculiar notching of the nucleus is often present and has been sug- 

 gested as characteristic of these cells, but as it is found in cells which 

 do not in other ways conform to this t^-pe and as it does not appear 

 to be present in all of them, I do not feel that this is a sure test. 



Of course the youngest embryos, i. e., those without a mesoderm, 

 22 



