46 CELL-DIVISION AND GROWTH II. i 



together by the vitelline membrane. Five surfaces of contact 

 were formed, that between two opposite bubbles representing 

 the cross furrow or polar furrow in the egg. In the Trochus 

 egg, however, the polar furrows need not be parallel at the 

 animal and vegetative pole ; they may be at right angles to one 

 another, and this tetrahedral arrangement of crossed polar 

 furrows may be imitated by lifting up one of the bubbles and 

 bringing it into contact with its opposite, one pair of bubbles 

 being now in contact below, the other pair above. This arrange- 

 ment is, however, unstable while the four bubbles remain in one 

 plane, the two bubbles soon coming into contact both above and 

 below. When the bubbles are not confined within a cup the 

 instability of the ' crossed-furrow ' condition is extreme. 



By reducing the volume of the bubbles that are in contact 

 the other two may be brought together ; as the polar furrow 

 changes positions there is at least a temporary condition when 

 they are crossed. 



As we have already pointed out, both conditions the ' parallel 

 furrows ' and the ' crossed furrows ' are met with in the eggs 

 at the four-celled stage of Molluscs, Annelids, and marine 

 Turbellarians. Whether both opposite pairs or only one opposite 

 pair of blastomeres are in contact does not, however, appear to 

 depend upon whether the vitelline membrane is close to and 

 compresses the egg or not. In most cases of crossed furrows 

 the membrane fits, it is true, quite closely (Nereis, IchnochUou, 

 Podarke, Lepidonotus, Discocelis, Physa, and possibly Limax and 

 Planorlis, if there is in these two, as in Physa, a very fine mem- 

 brane between the albumen and the ovum); so also, speaking 

 generally, where the furrows are parallel the membrane is absent 

 (Umbrella, Aplysia, Dreissemia, Crepitlula), but in Amphltrite and 

 Ctymenella it is lightly applied to the egg. 



It is remarkable that when the furrows are crossed, it is the 

 A and C cells which meet at the animal pole, the B and D cells 

 at the vegetative (except only in Uuio), and this must depend on 

 other properties of the cells than their surface tensions. But it 

 may be very plausibly suggested that the explanation of the fact 

 that it is the cells B and D which meet to make the ' parallel ' 

 furrows is to be looked for simply in the large size of D. 



Robert has indeed shown that by simply altering the sizes of 



