264 Robert Wilheliii liegiior. 



of the blastoderm-ecll (bl.c.) is closely packed with chromomeres, 

 and no nucleolus, such as is present in the pole-cell nticleus, is visible. 



When the second mitosis is completed, it is difficult to determine 

 the exact number of pole-cells, owing to their irregnlar arrange- 

 ment. A superficial view of the posterior end of the egg discloses 

 (Fig. 15) sixty-three pole-cells, one, the sixty-fourth, probably being 

 hidden from sight by overlying cells. In lateral surface view (Fig. 

 IG) the pole-cells are seen as a cap closely applied to the postei'ior 

 end of the egg. Longitudinal sections (Figs. 23-25) demonstrate 

 that they do not lie free in the polar cavity as described in Chiro- 

 nomus (Ritter, 1890, and others), but that those nearest the egg 

 occupy an indentation in its end, in which position they are probably 

 held by the vitelline membrane and the chorion (Balbiani, 1885, in 

 Chironotnus) . 



The entire surface of the egg, in the stage just described, is found 

 to be covered by the blastoderm, except the area at the posterior end 

 through which the primary pole-cells passed. Just within this area 

 are seen (Fig. 25) a number of nuclei similar to those of the neighbor- 

 ing blastoderm-cells ; they are not, as the latter, separated from one 

 another by cell boundaries, but form an irregular mass, a syncytium. 

 A group of nuclei similar in position and appearance has been noted 

 by Ritter (1890) in Chironomus and by Noack (1901) in CaUi- 

 phora; both authors maintain that these are yolk-nuclei. Lecaillon 

 (1898) figures them in an egg of Lina populi, but does not mention 

 them in the text (his Fig. 16). Voeltzkow (1889) found a few 

 nuclei in eggs of Musca lying in the same region. He calls them in- 

 wandering blastoderm-cells. I hope to prove that they are the nuclei 

 of blastoderm-cells, which, owing to the presence of the pole-cells, have 

 been prevented from taking part in the formation of the blastoderm. 

 In the stages figured (Figs. 6-11) no vitellophags are present near the 

 posterior end of the egg from which this group could h,ave arisen. 

 Furthermore, no similar groups of nuclei are tO' be found at other 

 places in the egg until a much later periled of development ; then, 

 there may be found irregularly scattered throughout the yolk, small 

 masses of cytoplasm, each containing thre(> or four yolk-nuclei. A 

 comparison of Figs. 23 and 24- shows that in the younger stages 



