JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKIL 91 
part of the material of the egg immediately surrounding the region of 
polar-cell formation is moved to the posterior end of the egg, and, later 
across this and even upon the dorsal side. The same is doubtless true 
for the corresponding region (opposite the place of polar-cell formation, 
according to Zelinka) in Callidina. It marks tho animal or upper pole 
in Asplanchna, lying at the opposite end of the egg from the blastopore 
during gastrulation, and is the common point of meeting for the blasto- 
meres derived from the four quadrants of the egg. In Callidina russe- 
ola, according to Zelinka, the region where the polar cell is formed lies, 
not at the opposite end of the gastrula from tho blastopore, but at the 
dorsal margin of the blastopore, and the cells of this region are lator 
invaginated to form the fundament of the pharynx; the real animal 
pole of the egg lying at a distance from the point of polar-cell forma- 
tion. The whole of Zelinka’s general discussion of the early develop. 
ment of the rotifer egg is based upon this peculiar position of the polar 
cell. (See Zelinka, ’91, pp. 132-135.) His general statement of the 
place of polar-cell formation is as follows: “ Das Richtungskörperchen 
kommt an der dorsalen Seite des künftigen Embryo hervor, bei Melicerta 
dem späteren hinteren Pole näher, bei Callidina fast am späteren vor- 
deren Pole des in beiden Fällen länglichen Kies.” 
The difference between our accounts is seen by comparing Figure 6 
(Plate 1) and Figure 8 (Plate 2) with Zelinka’s Figures 8, 9, and 10 
(Tafel J.). In the two-cell stage in Asplanchna (Fig. 6), when the 
smaller cell, AB’, is turned away from the observer, who looks down 
upon the polar cell, the spindle in the larger cell is seen to occupy such 
& position that the smaller produet of the division of OD’ will lie to 
the right, and in Figure 8 this condition is shown to be realized when the 
division has taken place, the cell C? lying to the right of the polar cell. 
3), derived from 
In Zelinka’s figures, however, the small cell II 
the division of the larger blastomere of the t» 
left of the polar cell, when the same orientation is 
of the two- 
the smaller blastomere (A 
observer. It therefore follows that, if the position of 
dorsal in Callidina, it must be ventral in Asplan 
orientation. Later stages show the same contrast. Thus Figures 13 
and 14 (Plate 2), representations of the eight-cell stage of Asplanchna, 
show the polar cell in the position already described, (the dorsal pole of 
the egg being directed toward the observer,) whereas Zelinka’s Figures 
15, 16, and 18 of Callidina show it at the opposite pole. Figures 19, 
20 (Plate 3), 38, 41 (Plate 5), and 59 (Plate 7) show the polar cell at 
