108 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
with the withdrawal of the deep parts of the peripheral cells and their 
dorso-ventral extension, consequent upon frequent equatorial divisions. 
19. As to facts bearing upon the special morphology of the Rotifera, — 
(a) The polar cell is formed at the animal pole of the egg, at the 
point opposite that where the blastopore is later found, and not at 
the dorsal (or anterior) margin of the future blastoporie region, as 
stated by Zelinka (91) for Callidina. 
(b) The cleavage of Asplanchna was traced to a later stage than has 
been done for other rotifers. A much greater regularity, and in a cer- 
tain sense symmetry, are shown in the direction aud rate of cleavage 
than has been shown for other species. 
B. Conclusions. 
20. It results from 1, 2, 3, 5, 7, and 9 that the direction of cleavage 
is not determined by any simple mechanical factors or relations of form. 
Specifically, the course of cleavage in Asplanchna is inconsistent with 
any general validity of (1) Hertwig's law of the spindles in the longest 
axis of the protoplasmic mass, (2) Berthold’s law of least areas, and 
(3) Braem’s and Pflüger’s principle of least resistance. 
21. It results from 11 that no simple factor can account for the 
equality or inequality of the cleavage. Specifically, the conditions 
in Asplanchna are inconsistent (a) with Hertwig's view that the 
dividing nucleus takes a position *in the middle of its sphere of 
action," so far as that expression has any definite significance, and 
(b) with Braem’s principle of “like resistance " at the two ends of the 
spindle. 
22. It results fron 12, as well as from a comparison with the cleavage 
of many other invertebrates, that no simple factor, such as greater or 
less quantity of yolk, will account for the sequence of cleavage. 
23. It is a natural conclusion from 15 and the latter part of 14, that 
the direction of the spindle is not due to an influence in the egg as! 
whole, connected with its axial relations, but is determined within each 
cell itself. However, I do not consider this conclusion at all well 
established. 
24. It results from 5, 6, 7, 8, and 9 that the problem as to what de- 
termines the position of the spindle is resolvable into several: (a) What 
determines the direction of separation of the newly formed asters? 
(b) What determines the position of the asters during the resting stage 
