JENXIXGS: DEVELOPMENT OF ASPLAXCHNA nEEEICKII. 



tion of cleavage, is determined, partially at least, by the same factors 

 ■which determine the form and relative position of soap bubbles in a mass. 

 As a result of surface tension, the cells take such forms as to occupy 

 the given space with the least possible surface areas. Xew septa will 

 appear in such positions that their surfaces will be the least possible 

 areas that could divide the cell into parts of the required size. " Die 

 Lamellensysteme ordnen sich so an, die einzeluen Lamellen kriimmen 

 sich in der Weise, dass die Summe der Oberllachen aller unter den ge- 

 geben Verhaltnissen ein Minimum wird." (Berthold, '86, pp. 219, 220.) 

 (2) Hertwig's law of the spindle in the longest axis of the protoplasmic 

 jnass. — According to Hertwig's well known view, as a result of the inter- 

 action of nucleus and protoplasm, the spindle during division comes to 

 lie in such a position that its longitudinal axis coincides with the axis 

 ■which passes through the greatest protoplasmic mass. " Es lasst sich 

 hier das zweite allgemeine Gesetz aufstellen, dass die beiden Pole der 

 Theilungsfigur in die Richtung der grossten Protoplasmamassen zu 

 liegen kommen, etwa in derselben AVeise, wie die Lage der Pole eines 

 Magneten durch Eisentheile in seiner Umgebung beeinflusst wird." 

 (Hertwig, '93, p. 175.) 



(.3) Braem's theory of separation in the direction of the greatest space 

 for development. — This is a modification of the principle of least pres- 

 sure, first enunciated by Pflliger ('8-i). Since Pflliger's principle, con- 

 sidered from a purely mechanical standpoint, seems irreconcilable with 

 the nature of the material on which it was supposed to act, and since 

 Braem's view is based on an essentially different conception of the na- 

 ture of the phenomena, I have not thought it necessary to take into 

 direct consideration Pfliigei''s view. 



Braem holds that when an egg is subjected to unequal pressure, the 

 spindle places itself in such a position that tlie resulting products shall 

 have the freest opportunity for development ; that is, in the direction of 

 'least resistance. The rule is not the expression of a purely mechanical 

 force, but is to a certain extent teleological in character. "Die Spindel 

 eines ungleichem Druck unterliegenden Eies stellt sich in derjenigen 

 Richtung ein, in welcher der riiumlicheu Eutfaltung der Zelle und ihrer 

 Teilprodukte der freieste Spielraum geboten ist. Ich glaube, dass diese 

 Fassung trotz oder vielmehr gerade wesen ihres teleologischen Gehaltes 

 dem TVesen der Sache besser entspricht als die rein mechanische Deu- 

 tung." (Braem, '94, pp. 341, 342.) 



The result is held to be due to a sort of sensory power resident in 

 the egg, "eine Art Tastsinn, durch den es der Zelle moglich wird, sich 



