60 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
due to surface tension at all, but to turgor. Turgor, however, can 
hardly be a factor in the cleavage of the egg, where no increase in size is 
taking place. 
(2) Hertwig’s law of the spindle in the longest axis of the protoplasmic 
mass. (Compare page 5.) — This is, so far as I know, the only principle 
for which the claim is made, that it is % decisive factor in determining 
the direction of the spindle. The statement quoted above (page 5) is 
from Hertwig's general text-book on the subject, in which it is presum- 
able that care would be taken not to mislead the reader unacquainted 
with the literature into taking a special phenomenon for a law of general 
import. On the same page it is stated that “ Mit diesen Regeln stimmen 
die Erscheinungen, wie sie bei der Zelltheilung und insbesondere bei der 
Eifurchung beobachtet werden, fast ausnahmslos überein." (Hertwig, 
93, p. 175.) "The “law” has been accepted by others in the same gen- 
eral bearing. Thus Ziegler (94, p. 154) questions the validity of cases 
apparently not in agreement with the rule, holding that they are due 
either to inexact study (cylindrical epithelium), or to the difficulties of 
determining in the presence of a mass of yolk (amphibian egg) which 2s 
the longest dimension of the protoplasmic mass. This principle has 
become the most widely known and generally accepted of any of the 
principles which have been proposed in regard to the determining factors 
in cell division. I shall therefore discuss it at somewhat greater length 
than Berthold’s principle, analyzing in detail the evidence on the subject 
from my own work, and reviewing that advanced by others. 
A comparison of the very first cleavage of Asplanchna with that of 
Callidina (Zelinka, '91) shows that in the Rotifera the form of the egg is 
not the factor determining the position of the first cleavage spindle. 
For in the two forms the first cleavage spindle bears the same relation to 
the animal pole, or place of polar-cell formation, but a different relation 
to the long axis of the egg. In Asplanchna the spindle at the time of 
division lies in the long axis of the egg (though a little earlier it is 
oblique), whereas in Callidina (Zelinka,’91, Taf. I. Fig. 5) the spindle at 
division is oblique to both the longer and shorter axes, — the place of 
polar-cell formation not being the same as in Asplanchna, but much 
nearer one end of the egg. The orientation of the spindle in these two 
rotifers, then, is constant with reference to the animal pole, but variable 
with reference to the form of the egg. 
The passage from the eight- to the sixteen-cell stage in Asplanchna is 
particularly instructive. The asters of the six cells of quadrants A, B, 
and C first separate in such a way that the line joining them lies in the 
