280 Robert Wilhelm Hegnor. 



the germ-cells separate into groups which migrate from a point near 

 the posterior end of the egg, to the fourth, fifth, and sixth abdominal 

 segments. 



As soon as the pole-cells of CalUgrapJta have passed through the 

 pole-cell canal, they loose their pronounced pseudopodia-like pro- 

 cesses and become nearly spherical (Fig. 55) ; nevertheless, they un- 

 dergo a decided change in position. They move away from the 

 inner end of the pole-cell canal, and creep along between the yolk 

 and the germ-band (Figs. 47-49). Thus two groups are formed 

 near the developing coelomic sacs ; each group probably contains 

 an equal number of cells. The smallest number I have counted in 

 one group at this time (Stage M) is thirty; the largest number, 

 thirty-four. As there is some difficulty in obtaining an accurate 

 count, it seems probable that the sixty-four germ-cells are equally 

 divided and that each germ-gland receives thirty-two. Some of the 

 germ-cells migrate not only laterally along the germ-band but also 

 back toward the posterior end of the egg, where we find tbem form- 

 ing narrow strands in the last abdominal segments (Figs. 39-41). 

 From this stage on, the germ-cells are not very active ; they move 

 closer to one another to form the compact germ-glands. I was unable 

 to determine whether the later movements of the germ-cells are due 

 to an active migration, or to the tensions created by the growth of 

 the surrounding tissues ; the latter seems the more probable. 



C. The Method of Locomotion of the Germ-Cells. 

 Nearly all of the authors, who have observed the migration of 

 the germ-cells in the Insecta, have failed to describe their method 

 of locomotion. Thus Robin (1862) and many of his contemporaries 

 state that in the Diptera, the pole-cells, shortly after their appearance, 

 move back into the egg. These authors, however, give no explana- 

 tion to account for these movements, and one is not enlightened 

 as to whether the pole-cells are passively carried from place to place, 

 or whether they undergo an active migration. That amceboid move- 

 ments might possibly explain the re-entering of the egg by the pole- 

 cells, was first suggested by Weismann (1882). He was unable to 

 follow the history of these cells in Chironomvs, because they dis- 



