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JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 19 
ends of the egg, but remain connected for a time by a distinct strand 
(Fig. 4). Meantime, before the first cleavage plane has appeared in the 
cytoplasm, the aster of the smaller blastomere has begun to divide, as 
shown in Figure 4. The two resulting asters separate at right angles 
to the axis of the first cleavage spindle. In the future larger cell the 
aster does not begin at once to divide. Both nuclei begin immediately 
to increase in size. The first cleavage plane passes through the point 
on the surface of the egg marked by the polar cell, transversely to the 
long axis of the ovum, and through about the middle of the strand con- 
necting the two nuclei. The strand is slightly thickened at the point 
where the first cleavage plane is to meet it (Fig. 4), indicating perhaps 
" The cleavage plane is thus 
the formation of the “ Zwischenkórper. 
perpendieular to the axis of the spindle, and passes through its middle. 
I mention this fact on account of the difference between the first cleay- 
age of Asplanchna and that of Callidina. In the latter rotifer, ac- 
cording to Zelinka (91), the first eleavage plane is oblique to the 
spindle, and the spindle itself, even at the time of division, is oblique 
to the long axis of the egg. In another rotifer, Eosphora, the first 
cleavage plane is likewise oblique to the long axis of the egg (Tessin, 
86), while in Melicerta ringens (Zelinka, 91) and Asplanchna Sieboldii 
(Lameere, 90) the first cleavage plane is transverse to the long axis, 
as in Asplanchna Herrickii. 
During and after the passage of the first cleavage plane through the 
cytoplasm, the egg retains its ellipsoidal form, and the resulting cells do 
not separate and become rounded, as occurs in the eggs of so many 
animals, but remain closely pressed together. In a large series of cases 
showing the first cleavage in various stages, the only indication of any 
change in the form of the egg or its blastomeres is a slight depression 
of the surface where the cleavage plane cuts the periphery of the egg, 
forming a shallow furrow. Here the edges of the two blastomeres are 
slightly rounded off, as shown in Figure 6, instead of fitting squarely 
against each other. The retention of its general form by the egg is 
characteristic of all cleavage stages. This surface of contact of the two 
blastomeres is curved, the smaller cell, AB’, projecting slightly into 
the larger. 
So far as the direction of the division is concerned, the first cleavage 
of Asplanchna evidently fits easily either the surface tension theory of 
Berthold, or Hertwig’s theory of the spindle in the long axis of the pro- 
toplasmie mass. Comparison with the first division of Callidina russe- 
ola as described by Zelinka (’91) develops an interesting fact. In 
