| 
| 
JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRIOKII. 5 
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. New 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 einzelnen Lamellen krümmen 
sich in der Weise, dass die Summe der Oberflächen aller unter den ge- 
geben Verhältnissen ein Minimum wird.” (Berthold, '86, pp. 219, 220.) 
(2) Hertwig’s law of the spindle in the longest axis of the protoplasmic 
mass. — 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 lässt sich 
hier das zweite allpemeine Gesetz aufstellen, dass die beiden Pole der 
Theilungsfigur in die Richtung der grössten Protoplasmamassen zu 
liegen kommen, etwa in derselben Weise, 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 Pflüger (84). Since Pflüger'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 
direet consideration Pflüger's view. 
Braem holds that when an egg is subjected to unequal pressure, the 
spindle places itself in such a position that the 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 räumlichen Entfaltung der Zelle und ihrer 
Teilprodukte der freieste Spielraum geboten ist. Ich glaube, dass diese 
Fassung trotz oder vielmehr gerade wegen ihres teleologischen Gehaltes 
dem Wesen 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 möglich wird, sich 
