94 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



tion is carried to the equator of the egg. The polar cell itself is shifted 

 at the same time, and it seems possible that during this rapid , rotation 

 it may be transferred to a region of the egg diflereut from its original 

 position. Such shiftiugs of the polar cell, though to a less extent, 

 are mentioned by Zelinka (pp. 55, 58). During the "sehr schnell ver- 

 laufendes Phiinomen" of the rotation of the egg within its shell, the 

 relatively diftereut shifting of the polar cell might have been over- 

 looked. This is, however, only the suggestion of a possibility, for which 

 there is no direct evidence in Zelinka's work. 



It is to be noted that in the above discussion I have employed 

 throughout the orientation of the egg used by Zelinka, and not that 

 adopted in my own account of the development. 



B. Cleavage. 



The first cleavage in the two species of Asplanchna differs from that 

 of Callidina russeola in heing exactly transverse to the long axis of the 

 egg. In the latter form, according to Zelinka, the first cleavage plane 

 is oblique to the spindle, and the spindle itself is oblique to the long 

 axis of the egg. By a change of their relative positions immediately 

 after division, the two cells ai'e later brought into the same position 

 relative to each other as in Asplanchna, and even at first the cleavage 

 plane in the two forms occupies the same position relative to the place of 

 polar-cell formation. The difference thus is of little importance, except 

 that, from a cyto-mechanical standpoint, it shows that the form of the 

 egg does not determine the position of the first cleavage spindle. In 

 Eosphora also (Tessin, '86) the first cleavage plane is oblique to the 

 long axis of tlie egg, whereas in Melicerta ringens (Zelinka, '91) and 

 Asplanchna Sieboldii (Lameere, '90) the first cleavage plane is transverse 

 to the long axis, as in Asplanchna Herrickii and Asplanchna priodonta. 



The second and third cleavages in the two species of Asplanchna 

 (Plate 1, Fig. 6, Plate 2, Figs. 8-16) are essentially similar to the 

 corresponding cleavages of Callidina and of other rotifers in which 

 the development has been described. For convenience in compar- 

 ing the later stages, I give here a table showing the correspond- 

 ence between the cells of Asplanchna in the eight-cell stage and those 

 of Callidina. 



Asplanclina. Callidina (Zelinka, '91). Asplanchna. Callidina (Zelinka, '91). 

 z*i a, c4i Ill 



112 

 I 



ni 



