No. I.] THE EMBRYOLOGY OF CREPIDULA. 



119 



Cdl5lAqeU 



2-4 



56 



64* (Figured butnot 



tocribed) 



^a- 



\ 





Za''(Ti(iCell) 



2. a''^ (Lower) 

 '^a.^-^(Lower 



{''"■<r^:\ 



■ z a '■^■'' 



Ng(it^"-^(krf.Cflls.l 



4< 



7 >n quAdranT D, 

 6 lacdch of Die. others 



Heymons ('93) has observed every division of this quartette 

 in Umbrella up to the formation of ten cells in each quadrant 

 (he merely mentions the last two divisions, and does not figure 

 or describe them). These divisions are cell for cell exactly like 

 those in Crepidula. Even the direction of evefy cleavage and the 

 size of all tJie resulting cells are the same. This wonderful and 

 long-preserved resemblance is more particularly shown in the 

 table of cleavages in Umbrella. 



Ceil Stage 1^ ^4 



3T 



48-55 



(0 



^a— 



f (Right) - 



(Left) 



aa'XTitiCell) 



\.i. 



/ 



2.a 



\. 2.. I. 



N^a'-^ 



.^a' 



2.. 1. 1. 





l.i. 





^ I.I.I. 

 /"Aft 



\2.a'-'-^ 

 va'-^-'-^ 



K^ of 2.0^ Quaif. Cells, i 



4 



6 



io 



With the exception of one division of the upper left cell in 

 each quadrant, Heymons has seen every division of this quar- 

 tette which I have observed in Crepidula. Inasmuch as he 

 merely mentions the fact that he has observed divisions of the 



