310 Origin and Migration of the Germ-Cells in Acanthias 



give difficnlty since the younger the embryo the more the cells in gen- 

 eral are filled with yolk sphernles and conform in other ways to the 

 germ-cell type. 



Let us follow the changes onward from the young blastoderm up to 

 the point of sexual differentiation of the genital gland in embryos of 

 34 mm. 



The first cut (Fig. 1) is taken through the greatest diameter of an 

 embryo of Balfour's stage A, at the growing point. The cells of the 

 ectoderm are somewhat differentiated, being long and slender, the other 



Fig. 2. 



Fig. 1. Blastoderm, Balfour's Stage A, at the growing point. Ec. ectoderm, En. 

 ebdoderm. Harvard Embryological Collection, Sag. Series, 490, section 81. x 330. 



Fig. 2. Blastoderm, slightly older than Balfour's stage A. Ec. ectoderm. En. 

 endoderm, Yk. yolk. x 330. 



or endodermal cells are nearly all alike and of such characteristics that 

 found in the mesoderm of a dog-fish 3-6 mm., I should unhesitatingly 

 call them primitive ova. No mesoderm is yet distinguishable. 



The second section (Fig. 2) is slightly older, still no mesoderm is 

 differentiated. Many of the cells of the endoderm show indefinite 

 limits in certain directions, long processes of protoplasm tending to join 

 them in a net work. Some of the cells, however, retain the earliest type. 



A section of a later age, Balfour's stage D, 2^ mm. long, shows the 

 mesoderm clearly formed on each side of the embryo. The split in the 



