No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 2:77 



turn slightly towards the right, and the cells themselves, as 

 they lengthen, undergo a twisting around in the same direction. 

 The first cleavage plane, when viewed from the animal pole, is 

 bent first to the left and then to the right. The reason of 

 this is that the cleavage is not perfectly horizontal, but two 

 of the cells lie a little higher than the others. When the 

 division is completed, there result four cells nearly equal in 

 size, two of which, B and D, come in contact below, in the 

 ventral cross furrow, while the other two, A and C, meet in a 

 cross furrow at the upper pole, which is nearly at right angles 

 to the lower one. The ventral cross furrow makes a negative 

 angle of about 45° with the first cleavage plane, while the 

 upper cross furrow makes with this plane an equal positive 

 angle. The bending of the first cleavage furrow at the lower 

 pole of the egg is the reverse of what occurs in Crepidula and 

 other mollusks with dexiotropic cleavage. It is a rule, holding 

 good for all known cases, that in dexiotropic cleavage the 

 ventral cross furrow, when viewed from the animal pole, bends 

 to the right, while in forms with reversed cleavage it bends to 

 the left. The cross furrow at the animal pole of the e.gg, 

 however, does not show such a constant relation to the first 

 cleavage plane. In many cases it may be absent entirely, the 

 four blastomeres meeting above in a point. In some cases it 

 is parallel to the ventral cross furrow (Crepidula), in others 

 it is nearly at right angles to it (Nereis, Planorbis, Physa, 

 Lymnaea). These variations obviously depend upon the fact 

 that in some cases the two cells B and D meet above as well 

 as below, — in which case the two polar furrows would be 

 parallel, — while in other eggs the alternate cells A and 67 meet 

 at the animal pole and form a cross furrow at right angles to 

 the lower one. It is not surprising, therefore, that the polar 

 furrows should sometimes present different relations to each 

 other in eggs of the same species. 



The four cells round off after division like those produced 

 by the previous cleavage. They become almost spherical, but 

 they subsequently draw together and assume very nearly the 

 form of a single undivided sphere. A cleavage cavity occurs 

 between the two pairs of cells AB and CD ; that is, between 



