I04 CON KLIN. [Vol. XIII. 



is not so easily explained. No phenomenon is more common 

 than the unequal division of apparently homogeneous cells, but 

 none is more difficult of explanation on the grounds of a purely 

 mechanical theory of development. All these general phenom- 

 ena might be perhaps the result of known mechanical conditions; 

 and yet the combinations, modifications, and coordinations of 

 these processes, which appear in the formation of almost any struc- 

 ture, ultimately require some explanation other than mechanics 

 can as yet supply. This is abundantly illustrated in the whole 

 history of the cleavage of such an egg as that which we are consid- 

 ering, and nowhere better than in the formation of the cross. 



Up to the time when there are two cells in each arm of the 

 cross, the position of each cell may be attributed, at least in 

 part, to the regular alternation in direction of successive cleav- 

 ages. The next step in the formation of the cross is highly 

 peculiar ; the basal cells, which were formed by dexiotropic 

 cleavage, divide in a dexiotropic direction in both Neritina and 

 Crepidula. In Umbrella, on the other hand, this division is 

 laeotropic, as it should be, according to the rule that successive 

 cleavages are in opposite directions. Associated with this 

 regular alternation of cleavage in Umbrella is the fact that 

 immediately before the division of the basal cells, the turrets 

 divide almost bilaterally ; whereas they remain undivided in 

 Neritina and Crepidula, in which the cleavage of the basals is 

 reversed. I was therefore inclined, at first, to attribute this 

 reversal to the lateral pressure of the undivided turrets upon the 

 basals, but several considerations have convinced me that this 

 cannot be the case ; in the first place the basals show no signs 

 of such pressure, being full, well-rounded cells ; again the turret 

 cells in Neritina are not large enough to exert any considerable 

 lateral pressure upon the arms of the cross ; and, finally, even 

 if such pressure were exerted, it would deflect the spindles 

 only a few degrees from the normal position, and would still 

 leave them laeotropic, whereas they are distinctly dexiotropic. 



The same considerations are applicable to the history of the 

 posterior arm, where repeated divisions are always in the same 

 direction, as is true of teloblastic growth in general, and also to 

 many of the later cleavages where reversals occur again and 



