JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 75 
throughout an equality in the size of the two products in all the cells. 
The two assumptions above stated are strikingly contradicted, not only 
in the formation of the polar cell (Fig. 1), but also in the fifth (Fig. 
35, Plate 5) and sixth (Fig. 49, Plate 6) cleavages of the entodermal 
cell (di. and de-), and in the division of the entodermal cell d*? (Fig. 
80, Plate 10). In these cases the spindle moves against one side of th 
cell, and there a small vesicle is produced, the two products of cleavage 
being exeessively unequal. "The contradiction is emphasized by tho fact 
that in the fifth and sixth cleavages of the entodermal cell (Figs. 35, 
48, and 49), although there is a concentration of yolk in ono region of 
the cell, it bears no significant relation to the position of the spindle, and 
by the further remarkable fact that in the next or seventh cleavage 
(Figs. 64 and 65, Plate 8), this yolk cloud passes into the smaller 
blastomere. 
When one considers the method of formation of polar cell in all eggs, 
and the almost unlimited range in the comparative size of cells resulting 
from cleavage in a great number of organisms, the grounds for the 
generalization, that “der Kern stets die Mitte seiner Wirkungssphüre 
einzunehmen sucht,“ are ditficult to comprehend, at least so far as they 
relate to the dividing nucleus. As a statement of fact for specific cases, 
with the word “stets” omitted, I have no fault to find with it; but as a 
generalization it is, so far as it relates to the dividing nucleus, meaningless. 
The position of the spindle within the cell must be considered to be re- 
lated to the purpose of the ensuing division. It is of course probable 
that in many eases the primary purpose is to divide the formative proto- 
plasm equally botween the two products, and this may determine the 
position of the spindle in, for example, the first, second, and third 
cleavages of the frog's egg. But what determines the position of the 
spindle in the two divisions immediately preceding these, — in the forma- 
tion of polar cells? It seems to have been generally overlooked, even by 
those who have pointed out that polar-cell formation is cell division, that 
the formation of polar cells must be reckoned with in any general theory 
of cell division. 
An examination of the cleavage of many invertebrates shows that, as 
in Asplanehna, equal eleavage is no more the rule than unequal cleavage, 
even where there is no corresponding differentiation into yolk bearing 
and purely protoplasmie regions. The statement that the dividing 
nucleus tends to take a position in the middle of its sphere of action is 
true, if we consider the “middle of its sphere of action” to be, as it 
actually is, the point where it divides. But with this interpretation the 
