JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 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 the 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 the blastopore, but at the 

 dorsal margin of the blastopore, and the cells of this region are later 

 iuvaginated 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 Richtungskorperchen 

 kommt an der dorsalen Seite desktluftigen Embryo hervor, bei Melicerta 

 dem spateren hinteren Pole naher, bei Callidina fast am spateren vor- 

 deren Pole des in beiden Fallen langlichen Eies." 



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 I.). 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 

 a position that the smaller product of the division of Glf will lie to 

 the right, and in Figure 8 this condition is shown to be realized when the 

 division has taken place, the cell G^ lying to the right of the polar cell. 

 In Zelinka's figures, however, the small cell II (= (7*), derived from 

 the division of the larger blastomere of the two-cell stage, lies to the 

 left of the polar cell, when the same orientation is adopted ; i. e. with 

 the smaller blastomere (A = AB') of the two-cell stage away from tlie 

 observer. It therefore follows that, if the position of the polar cell is 

 dorsal in Callidina, it must be ventral in Asplanchna, using Zelinha^s 

 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, IG, 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 



