The Development of IschnochitOD. 601 



The median cells of the cross may also appear in a state of 

 division at this time but they usually cleave after the division of the 

 basais. The cleavage is leiotropic and also results in two cells of 

 about the same size in each quadrant. 



From this time forward cleavages in these cross cells occur with 

 considerable irregularity. I have chosen such a stage as is shown in 

 Fig. 31 because it gives an accurate idea of the direction of the 

 cleavages but as a rule they do not occur so simultaneously. Con- 

 sidering the division of the two basal cells of each quadrant , the 

 cleavages are dexiotropic and leiotropic producing, in all but the 

 posterior quadrant, relatively smaller products (/ «j -Z. -2. i. ^. i. i and 

 10,1.2.1.1.2.1 etc.) than the parent cells. In each quadrant they lie 

 on either side of cell la^-^-^-^^ etc. of the rosette series and in 

 Fig. 35 the position and relative size of these cells in A und C is 

 shown. In the anterior quadrant they are about half the size of those 

 just described but the position is the same. In the posterior quadrant 

 the cells are of the same size as the parent cells and this gives rise to 

 a row of four uniformly sized cells that afford a ready means of 

 orientation (Fig. 34). 



Regarding the median cells, the cleavages are even more irregular 

 and the size of the resulting blastomeres is not constant, although 

 the direction of the cleavage is regular being dexiotropic as is shown 

 in Fig. 31 ; also in many cases more or less shifting results after the 

 cells are formed. 



The study of the cell lineage is impossible from this time on, 

 and it has probably reached a point when it ceases to possess any 

 value in a comparative way. The arms of the cross retain their 

 original position and in the subsequent development form the ecto- 

 derm of the head vesicle. 



Comparisons. 



Before leaving this phase of the development it may not be 

 without interest to compare it with what obtains in Crepidula. 



Neglecting the division which normally forms the accessory 

 trochoblast, the cross in Ischnochiton is precisely similar to that in 

 Crepidula up to the time when there are three cells in each arm. 

 In the latter form the first differences arise in the posterior quadrant 

 which will be considered later. In the remaining quadrants the first 

 division of the basais is identical in each case and the cells formed 

 by the cleavage of the tip and median cells have much the same 



39* 



