404 ROBERT W. HEGNER 



position as pole-cell antecedents; (2) these four nuclei divide 

 producing eight daughter nuclei which move closer to the pe- 

 riphery of the egg; (3) these in turn divide resulting in sixteen 

 nuclei, arranged in pairs, each of which separates entirely from 

 the egg, carrying with it a portion of the Keimhautblastem 

 containing pole-disc granules (fig. 7, C) ; (4) the sixteen primary 

 pole-cells divide to form thirty-two secondary pole-cells; these 

 divide resulting in sixty-four tertiary pole-cells which do not 

 increase in number until a late period of embryonic life; (5) 

 in mitosis the pole-disc granules are approximately equally 

 distributed between the two daughter cells (fig. 8, B). After 

 separation from the egg the pole-cells are (1) carried slightly 

 forward on the ventral surface of the egg by the contraction 

 of the ventral plate; (2) they sink into the posterior depression 

 of the ventral groove, which is the beginning of the posterior 

 amniotic cavity; (3) they are carried along by the developing 

 tail-fold, which penetrates dorso-anteriorly into the yolk; (4) 

 they migrate through a pole-cell canal into the embryo by means 

 of amoeboid movements; (5) upon reaching the interior of the 

 embryo they separate into two groups, which come to lie, one 

 on either side of the body, in the last two abdominal segments; 

 (6) these two strands become shorter by a crowding together 

 of the germ-cells; (7) each of the two germ-glands thus pro- 

 duced acquires an epithelial covering of mesoderm-cells; (8) the 

 germ-glands, situated as before in the last two abdominal seg- 

 ments, are carried, by the shortening of the embryo, to a ventral 

 position on either side of the body; (9) by its lateral growth 

 around the yolk, the embryo carries the germ-glands to a point 

 near the dorsal surface on either side of the mid-gut; (10) the 

 sexes can be distinguished at this time by the shape of the germ- 

 glands, that of the male being dumb-bell shaped, while the 

 female reproductive organ is pear-shaped, and shows the de- 

 velopment of terminal filaments. 



In all stages the germ cells may be distinguished easily from 

 the surrounding somatic cells. Figure 8, A shows a pole cell 

 shortly after separation from the egg. The pole-disc granules 

 are quite conspicuous, and pseudopodia-hke projections are 



