JENNINGS: DEVELOPMENT OF ASPLANCHNA HEREICKII. 19 



ends of the egg, but remain conuected for a time by a distinct strand 

 (Fi"-. 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 sepai'ate 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 formation of the " Zwischenkorper." The cleavage plane is thus 

 perpendicular to the axis of the spindle, and passes through its middle. 

 I mention this fact on account of the difference between the first cleav- 

 age of Asplanchna and that of Callidina. In the latter rotifer, ac- 

 cording to Zelinka ('91), the first cleavage 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, 

 '8G), 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 

 Bsrthold, or Hertwig's theory of the spindle in the long axis of the pro- 

 toplasmic mass. Comparison with the first division of Callidina russe- 

 ola as described by Zelinka ('91) develops an interesting fact. In 



