MIGRATION OF SEX-CELLS OF FUNDULUS HETEROCLITUS. 33! 



opment of the embryo at the regions represented. The sex-cells 

 lie between the periblast and the endoderm in Fig. 10 ; while in 

 Fig. 9 their position is below the mesoderm and lateral to the de- 

 veloping hind gut. 



The positions of the most anterior sex-cells in embryo B' 23 are 

 indicated in Text Fig. A. The rectangle in this text figure in- 

 cludes the region which is drawn in detail in Fig. 8. Here a pri- 

 mary sex-cell is shown which is entirely free from any possible 

 connection with the lateral mesoderm. It can scarcely be said to 

 lie in, but rather lateral to the gut endoderm. It is half buried in 

 the periblast. This fact suggests intimate relation with this nu- 

 tritive layer. The cell figured is one of the few ever found with 

 an irregular outline. This might seem to suggest amoeboid activ- 

 ity, but this type is so extremely rare that it may be neglected 

 from consideration. 



Fig. 13 from B' 23 shows a sex-cell which is .06 mm. to the rear 

 of the one just mentioned. It is plainly in that portion of the 

 lateral mesoderm which will develop into the splanchnic layer 

 upon the formation of the ccelome (about the third day). 



Observations of these early embryos show several important 

 facts. The primary sex-cells are as truly characteristic and as 

 easily recognizable as any found in the germ glands of later stages. 

 They are located in the posterior half of the embryo, becoming 

 gradually more numerous as the anterior part of this region is 

 approached. Laterally they range from the extra-embryonic 

 region to within the lateral mesoderm and the edge of the de- 

 veloping gut. In general their progress along the germinal path 

 is directly proportional to the development of the embryo. 



lOj-Hour Embryo. Text Fig. B shows the relative positions 

 of the sex-cells in the 105-hour embryo, B' 26. As in previous 

 cases the rectangle indicates the area from which Figs. 14, 15 and 

 16 were drawn. These three figures from the same embryo illus- 

 trate the full extent of the migration at this stage. On the left 

 side of the embryo the sex-cells are found scattered all along the 

 splanchnic mesoderm, from the region very near the split in the 

 lateral mesoderm (Fig. 14) to that at the side of the gut (Fig. 

 16). On the right of Text Fig. B the sex-cells on the opposite 

 side of the embryo are shown massed lateral to the gut. Should 



