l82 



CONKLIN. 



[Vol. XIII. 



In the following table the reversals in Crepidula are classified 

 according to quadrants : 



Table of Reversals of Cleavage in Crepidula. 



Dropping all doubtful cases of reversal, in which the spindles 

 are nearly meridional or equatorial in position, there remain 

 three cases in quadrant A, three in C, four in B, and seven in D, 



To this table of reversals there should be appended a state- 

 ment that eight divisions which occur in the anterior quadrants, 

 A and B, are not represented in quadrant D, and likewise two 

 are not represented in quadrant C. All of these omitted cleav- 

 ages except two (3c'-' and 3d'-') are connected with the peculiar 

 history of the posterior turret cells and the posterior arm of 

 the cross. 



In a few cases which are classified here as reversals the 

 nuclear spindle does not indicate that the cleavage is to be 

 reversed, and even the daughter nuclei may occupy the same 

 relative positions as in the quadrants in which there is no 

 reversal, while at the same time the lobing of the cytoplasm 

 and the subsequent rotation show that the cleavage is reversed ; 

 the first division of 3d (Figs. 25, 26, 29) is a case in point. In 

 such cases the conditions which influence the direction of the 

 cleavage are not manifested until after the nuclear division is 

 completed, whereas they are usually shown in the direction of 

 the nuclear spindles and in the earliest stages of cleavage. 



