CLEAVAGE AND DIFFERENTIATION. 



23 



upon the direction of certain cleavages and ultimately upon the 

 first cleavage, these diverse planes of symmetry come to coin- 

 cide in a common plane. The direction of the asymmetry of 

 the adult Crepidula is also referable to the time and direction 

 of certain cleavages (of the fifth quartette) v^hich are explained 

 in part by the direction of preceding divisions and finally by 

 the direction of the first cleavage ; whereas in certain sinistral 

 gasteropods, as Crampton ^ and Kofoid ^ have shown, the direc- 

 tion of all the cleavages is reversed. 



All of these important and determinate characters are directly 



Fig. 2. — Crepidula, third cleavage; early indications of a dexiotropic rotation. 



referable to certain peculiarities of the unsegmented ^gg, and 

 although it is not possible to trace all determinate characters 

 to this early stage, yet it is highly probable that many others 

 are due to the same cause. How suggestive in this connection 

 are the observations of Blochmann ^ upon the Urvelarzellcn of 

 Neritina; these cells contain a mass of coarse granules which 

 can be traced back through previous generations of cells until 



1 Crampton, H. E., "Reversal of Cleavage in a Sinistral Gasteropod," Ann. New 

 York Acad. Sciences, VIII, 1S94. 



2 Kofoid, C. A., " On Some Laws of Cleavage in Limax," Proc. Am. Acad. Arts 

 and Sciences, XXIX, 1894. 



3 Blochmann, F., " Ueber die Entwicklung der Neritina fluviatilis," Zeit. wiss. 

 Zool., Bd. 36, 1 88 1. 



