JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRIOKII. 18 
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 first 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 surface, 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 polw cell a flattened, disk-like body, not projecting above the 
general surfaee 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 
mechanies, 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 
