JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 7 
to be at first in a line at right angles to the axis of the preceding spin- 
dle; then, by a rotation through an angle characteristic for the tissue, 
the definitive position is reached. The position which the spindle is 
finally to take is therefore determined at the time the asters separate. 
“Soweit ich indessen die Lage übersehen kann, ist die schliessliche 
Stellung der Spindel von dem Moment an fest gegeben, in welchem 
die Theilung des Muttermikrocentrums stattfand." (Heidenhain, ’95, 
pp. 555, 556.) 
(6) Sachs’s view, that the walls separating the cells meet one another at 
right angles. — This (Sachs, ’78, p. 1070) can hardly be considered as 
more than a statement of a condition commonly found, Berthold (’86, 
p. 252) and Hertwig (793, p. 177) have endeavored to show that the 
condition is explainable as a result of the theories proposed by them. 
(7) Rauber (83, p. 276) holds that there is evidence that the asters of 
the different blastomeres exercise an attraction for each other in such a way 
that, in a given area-composed of a number of cells, the spindles must 
take such positions as to bring about a condition of equilibrium among the 
asters. — “ Beurtheilt man die Verschiedenheiten der Furchennetzes von 
der Stellungen der karyokinetischen Achsen aus, so gewinnt es den 
Anschein, als ob die neu entstehenden Centren eines Blastomers auf 
diejenigen der angrenzenden Blastomeren einzuwirken vermógen und die 
Richtung ihrer Achsen beeinflussen." (Rauber, ’83, p. 280.) 
(8) Braem’s principle of equal resistance at the two ends of the spindle. 
— Subordinate to his principle of least resistance, Braem holds that the 
spindle tends to take such a position that the pressure at the two ends is 
the same. “Es ist das Prineip des gleichen Widerstandes, wodurch die 
horizontale Lage der Spindel bedingt wird. Wir müssen annehmen, dass 
der Kern von vornherein das Bestreben hat, sich gleichmässig nach bei- 
den Seiten hin auszudehnen und somit auf eine äquale Zellteilung 
hinzuwirken.“ (Braem, ’94, p. 345.) 
In the following deseription these theories will be kept in mind, and 
the bearing of the observations upon them pointed out. It will appear 
that, for certain of the theories, the conditions in the egg of Asplanchna 
present crucial tests. 
B. Equality or Inequality of Cleavage. 
The second aspect under which cleavage is determined is with regard 
to the relative size of the two products. What is it that determines 
whether the division shall be equal or unequal ? 
Concerning the factors which determine the equality or inequality of 
