31-i Origin and Migration of the Germ-Cells in Acanthias 



The larger mass of cells on the right shows the region in which most 

 of the sex cells are scattered. It will be observed that even yet the 



genital fold has not formed. There are 

 in this embryo 154 primitive ova in 

 the region ventral to the mesentery. 

 Seventy-three are in the mesentery and 

 69 in the genital region. 



Fig. 12 is a cross-section throngh an 

 11.5 nim. embryo. It shows the great- 

 est height of the embryo above the yolk 

 and also the corresponding length of 

 the intestine. The black dots indicate 

 the position of the primitive ova and 

 the relatively large space that they now 

 cover. About half of them would be 

 found in the mesentery at this stage; 

 137 were counted in this structure. 



Fig. 13 is a cross-section through a 

 15 mm. embryo at a point a little pos- 

 terior to the last in a region where the 

 connection of the embryo with the yolk 

 does not show. In such an embryo 

 most of the germ-cells are in or near 

 their ultimate position in the thickened epithelium near the letter d. 

 The great accumulation in the mesentery has been transferred to the 

 posterior coelom epithelium. There were 29 in the 

 mesentery and 41 ventral to this. 



Fig. 14 shows a cross-section of a 19 mm. em- 

 Tjryo. At this stage most of the cells have reached 

 the region of the genital fold. Many, however, 

 are at the root or posterior portion of the mesen- 

 tery, and a few are still in the mesentery itself. 



Out of 272 primitive ova, 242 were in the genital 

 fold now shown on the left or just to the median 

 side of it. There were 19 in the mesentery and 11 

 ventral to the mesentery. 



In an embryo of 28 mm. we see the genital gland 

 formed, and of 473 cells present, 469 were com- 

 pletely housed in the gland itself. The remaining four were close at 

 hand, being in the root of the mesentery. An embryo of 34 mm. was 



Fig. 10. Germ cpIIs as they appear 

 in a 6 mm. embryo as Vig. 9. Ec. 

 ectoderm, En. endoderm, Mes. meso- 

 derm, Ur. g-erm-colls. X 330. 



Fig. 11. Embryo of 8 

 mm. Most of the germ- 

 cells are near the point 

 Ur. En. endoderm. 

 Harvard Embrj'olog-i- 

 cal Collection. Trans. 

 Series 447. X 38. 



