60 bulletin: museum of comparative zoology. 



due to siirfixce tension at all, but to turgor. Turgor, however, can 

 hardly be a factor iu the cleavage of the egg, where no increase in size is 

 taking place. 



(2) Hertioig'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 the 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 wei'den, fast ausnahmslos iiberein." (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 is 

 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. 1 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 (Zeliuka, '91) shows that in the Rotifera the form of the egg is 

 not the factor delermining 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 eQ'g. 



The passage from the eight- to the sixteen-cell stage in Asplanchna is 

 pai-ticularly 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 



