JENNINGS: DEVELOPMENT OF ASPLANCHNA HEURICKII. 97 



variation as to the relative time of cleavage of two blastomeres (d^-^ aud 

 (/■'•^j, — a phenomenon which is exceedingly common both in differ'eut 

 individuals of the same species and in closely related species, and to 

 which little or no significance can be attached. It is Callidina which 

 shows a " bemerkenswerthe Verschiedenheit," since here the rhythm 

 aud regularity of the cleavage are completely destroyed, if the division 

 is coiTCctly described by Zelinka.^ 



Beyond the fourth cleavage it becomes very difficult to compare the 

 processes in Asplanchna with those described by other observers for 

 other rotifers. As above described, even in the fourth cleavage one of 

 the cells of the D quadrant (d^-^) was formed in a different manner in 

 Callidina, according to Zeliuka, from the method in Asplanchna. This 

 in itself makes an exact comparison of the fifth cleavage in the two 

 species impossible. But, considering for convenience the cells corre- 

 sponding in position at the sixteen-cell stage as equivalent in the two 

 forms, we find the following to be the process in Callidina as described 

 by Zelinka. 



The first blastomere of the sixteen-cell stage to divide in Callidina is 

 said to be the dorsal cell of the D quadrant, d^-* (III, Zelinka), whereas in 

 Asplanchna the very unequal division of the ventral cell d^-^ takes place 

 first (Fig. 33, Plate 4, and Figs. 35 and 38, Plate 5). The cleavage of 

 d^* is followed in Callidina by the division of d^-^ and d^-^ (IV and Y., 

 Zelinka). The division is meridional, as in Asplanchna (Fig. 37), but the 

 products are equal, whereas in Asplanchna they are unequal. 



Xow, according to Zelinka, the products (at the fifth cleavage) of the 

 division of d^-* {d^-'' and d'^-^, Zelinka's IIIi and IIL) are themselves 

 divided by meridional furrows. Thus the sixth cleavage takes place in 

 these cells before the fifth has been accomplished in any of the other 

 quadrants. 



Following this, the large ventral cell of the D quadrant, d^-^ (I, 

 Zelinka), divides equatorially, giving off on its dorsal side a small cell, 

 YI, which lies between the products of the division of d^'^. 



Thus the cleavage of the quadrant D is much less regular than in 

 Asplanchna, where the ventral cells all cleave meridionally and unequally, 

 the dorsal cell equatorially and unequally, the direction of cleavage 



1 Zelinka's statement, quoted above, that Melicerta follows the same method of 

 division as Callidina, depends merely upon his interpretation of a real variation ; 

 the actual divisions to form the sixteen-cell stage are the same in Melicerta as in 

 Asplanchna and Eosphora, and different from those of Callidina, as may be seen 

 by consulting Zelinka's ('91, p. 121) description. 



VOL. XXX. — NO. 1. 7 



