66 



CONKLIN. 



[Vol. XIII. 



true of several other gasteropods which I have studied. I think, 

 therefore, that Blochmann's derivation of these cells can no 

 longer be called in question. But it is evident, as Kofoid 

 points out, that he has made a mistake in the derivation of the 



•5i 3co. 



.•5b 



a 



5 



Diagram 5. — The 24-celI stage in Neritina and Crepidula. The nomenclature is the one used 

 throughout tliis paper. — a, The derivation of the belt cells of Neritina according to Eloch- 

 mann. The stippled cells give rise to the " Urvelarzellen." — c, Kofoid's modification of 

 Blochmann's account. — b. The derivation of these cells in Crepidula. The reference lines and 

 letters in a and c indicate the modifications necessary to bring Neritina into agreement with 

 Crepidula. 



belt cells. In the accompanying diagrams I give Blochmann's 

 Fig. 48, Kofoid's modification of this, and a corresponding 

 stage in Crepidula ; the nomenclature in each case has been 

 reduced to the system used in the present paper. 



