JENNINGS: DEVELOPMENT OF ASPLANCHNA HERPJCKII. 75 



throughout an equality in the size of the two products iu 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 {d^'^ and d^'^), and in the division of the entodermal cell d^-'^ (Fig. 

 80, Plate 10). In these cases the spindle moves against one side of th j 

 cell, and there a small vesicle is produced, the two products of cleavage 

 being excessively unequal. The contradiction is emphasized by the fact 

 that in the fifth and sixth cleavages of the entodermal cell (Figs. 35, 

 48, and 49), although there is a concentration of yolk iu one region of 

 the cell, it bears no significant relation to the position of tlie spindle, and 

 by the further remarkable flict tliat 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 iu 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 Wirkungssphilre 

 einzunehmen sucht," are difficult 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 pi'obable 

 that in many cases the primary purpose is to divide the formative proto- 

 plasm equally between 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 1 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 Asplanchna, equal cleavage is no more tlie rule than unequal cleavage, 

 even where there is no corresponding diff'erentiation into yolk bearing 

 and purely pi'otoplasmic 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 



