JENNINGS : DEVELOPMENT OF ASPLANCHNA HERPJCKII. 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 iudessen die Lage ubersehen kann, ist die schliessliche 

 Stellung der Spindel von dem Moment an fest gegeben, in welchem 

 die Theilung des Muttermikroceutrums stattfand." (Heideuhaiu, '95, 

 pp. 555, 556.) 



(6) Sachses 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 ('93, p. 177) have endeavored to show that the 

 condition is explainable as a result of the theories proposed by them. 



(7) Rauher ('83, p. 276) holds that there is evidence that the asters of 

 the different blastomeres exercise an attraction fur 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 Yerschiedenheiten der Furchennetzes von 

 der Stellungen der karyokinetischen Achsen aus, so gewinut es den 

 Anschein, als ob die neu entstehenden Centren eines Elastomers auf 

 diejenigen der angrenzenden Blastomeren einzuwirken vermogen und die 

 Eichtung 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 Priucip des gleichen Widerstandes, wodurch die 

 horizontale Lage der Spindel bedingt wird. Wir miissen annehmen, dass 

 der Kern von voruherein das Bestreben hat, sich gleichmassig nach bei- 

 den Seiten hin auszudehnen und somit auf eine aquale Zellteilung 

 hinzuwirken." (Braem, '94, p. 345.) 



In the following description 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 fixctors which determine the equality or inequality of 



