No. I.] THE EMBRYOLOGY OF CREPIDULA. 173 



Crampton's ('94) observation that in a sinistral gasteropod, 

 Physa, there is a reversal of the usual directions of spiral cleav- 

 age, is particularly interesting in this connection. If the initial 

 asymmetry is caused in Physa, as in Crepidula, by the asymme- 

 try of the cells 5C and 5D, then it is easy to see how this rever- 

 sal of the cleavage stands in a causal relation to the reversed 

 asymmetry of the adult. Both Crampton and Kofoid ('94) call 

 attention to the fact that in Rabl's ('79) figures of the embry- 

 ology of Planorbis there is a reversal of the usual direction of 

 cleavage, and also that in Haddon's ('82) figures of Janthina a 

 similar reversal is indicated. In Planorbis the asymmetry of 

 the adult is reversed, though this does not seem to be the case 

 in Janthina {cf. Kofoid ('95), p. 69). 



If it should prove on further investigation that reversed 

 cleavage always leads to reversed asymmetry of the adult, there 

 would be good reason for believing that Crepidula exhibits the 

 usual and perhaps the ancestral method in the establishment 

 of the asymmetry of the gasteropods. 



D. GENERAL CONSIDERATIONS. 

 I. The Forms of Cleavage. 



Several different kinds of cleavage are commonly recognized : 

 (i) with reference to its extent, cleavage is total (holoblastic) 

 or partial (meroblastic) ; (2) regarding the relative size of the 

 cell products it is equal or unequal ; (3) in the distribution of 

 yolk it is telolecithal or centrolecithal ; (4) with reference to 

 the constancy of form, it is regular or irregular ; (5) with 

 reference to symmetry it is radial or bilateral. To these five 

 classes I think a sixth should be added, at least for the present, 

 viz., one with reference to the destiny of the cleavage cells and 

 axes, and for which I propose the names determinate and 

 indeterminate. It may be that future work will show that this 

 distinction is not necessary, but in the present state of knowl- 

 edge such a distinction exists and it is very useful to have a 

 name for it. Attention is directed in this place only to the two 



