JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 97 
variation as to the relative time of cleavage of two blastomeres (& and 
d*?) — 4a phenomenon which is exceedingly common both in different 
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 
and regularity of the cleavage are completely destroyed, if the division 
is correctly 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 deseribed, even in the fourth eleavage one of 
the cells of the D quadrant (45.8) 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** (ITI, 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 
d5* is followed in Callidina by the division of d*? and d5? (IV and V., 
Zelinka). The division is meridional, as in Asplanchna (Fig. 37), but the 
products are equal, whereas in Asplanchna they are unequal. 
Now, according to Zelinka, the products (at the fifth cleavage) of the 
division of d9* (d. and des, Zelinka’s IIT, and IIIa) 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** (J, 
Zelinka), divides equatorially, giving off on its dorsal side a small cell, 
VI, 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 clenve 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 
Asplanehna and Eosphora, and different from those of Callidina, as may be seen 
by consulting Zelinka's ('91, p. 121) description. 
VOL. XXX. — NO. 1. 
