108 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



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 blastoporic 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 and 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 Pfliiger'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 a 

 ■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 



