JEXXINGS: DEVELOPMENT OF ASPLAXCHNA HERPJCKII. 13 



In immature eggs the large germinative vesicle is commonly found in 

 an eccentric position, with no very apparent relation to the axes of the 

 egg. In Asplanchna Sieboldii, according to Lameere ('90), just before 

 the movement of the germinative vesicle toward the spot where the 

 polar cell is formed, it lies in the long axis of the egg nearer one 

 pole, — in the position where later the 6rst cleavage spindle is located. 

 This is probably the case also in Asplanchna Herrickii ; but before 

 the germinative vesicle has begun to show the changes indicating 

 the formation of the maturation spindle it is difficult in preserved 

 material to get evidence as to the proper sequence of the stages 

 observed. 



Just before the maturation spindle is formed, the nucleus moves 

 toward the periphery of the egg, and begins to lose its spherical shape. 

 It takes a position close to the suiface, not at the equator, but nearer 

 one of the poles of the egg, as shown in Figure 1. In cases where the 

 differentiation into a more pointed pole and a blunter one is visible, 

 the nucleus always lies nearer the more pointed pole. Here a spindle is 

 formed, and the maturation division takes place. The polar cell thus 

 formed does not lie upon the outer surface of the egg as a free body, 

 but from the first is pressed into the substance of the yolk (Fig. 2), 

 as if by a firm membrane, in the manner described by Lameere for 

 Asplanchna Sieboldii. The nucleus begins to withdraw from the 

 periphery, at the same time resuming the spherical form, leaving 

 the polar cell a flattened, disk-like body, not projecting above the 

 general surface of the egg. This condition of the egg is shown in 

 Figure 2. 



From the first, therefore, the polar cell is imbedded in the substance 

 of the egg, so that it cannot suffer displacement during the processes 

 which follow. As will be shown in the course of this paper, the place 

 where the polar cell is formed marks the point on the surface of the egg 

 opposite to that at which gastrulation takes place. This is contrary to 

 the statement made by Zelinka ('91) for the egg of Callidina, and con- 

 trary to his general statement for the Rotifera as to the relation of the 

 place of polar cell formation to the later axes of the embryo. As this 

 matter is not of especial interest from the standpoint of developmental 

 mechanics, a full discussion of the difference between my account and 

 that of Zelinka is reserved for Part Second. There it will be shown by 

 evidence from Zelinka's own work, as well as that presented here, that 

 his general statement of the relation of the place where the polar cell is 

 formed to the axes of the egg in the Rotifera cannot be considered true 



