No. I.] THE EMBRYOLOGY OF CREPIDULA. 179 



stage, and even after this they continue in a majority of the 

 cells as long as the cleavage can be followed. 



Kofoid ('95) has collated from the most important literature 

 on spiral cleavage the facts as to alternation, and has presented 

 them in a series of excellently constructed tables. In con- 

 formity with the methods used by him, there are given in the 

 following table (pp. 180, 181) the facts as to the alternation 

 of successive cleavages in Crepidula. 



This table shows one very clear case of the reversal of an 

 entire spiral cleavage, viz., the division of the basal cells of the 

 cross la'--, etc.. Gen. VII. Two other cases, not so clearly 

 marked because the spindles are nearly meridional or equatorial, 

 are found in the division of the cells 2a^', etc.. Gen. VIII, and 

 the descendents of these cells, 2a^'-', etc., Gen. IX. 



There are many cases in which reversals are seen in one 

 quadrant, while the usual direction is preserved in the other 

 three. Thus, every division of the third quartette (with the 

 possible exception of 3a'% etc.) shows reversal in at least one 

 quadrant, and the same is true of certain cells of the first and 

 second quartettes. These reversals, however, unlike those 

 which occur in all four quadrants, have reference to the appear- 

 ance of bilateral symmetry. 



In Neritina there is a total reversal of the cleavage in the 

 basal cells of the cross ia'% etc., just as in Crepidula, whereas 

 in Umbrella the cleavage of these cells follows the usual rule. 

 I have elsewhere (p. 95) pointed out the fact that upon this 

 reversal the continued existence of the cross as a recognizable 

 structure in Neritina and Crepidula depends. In Neritina the 

 second division of the third quartette (3ai, etc.) is indicated in 

 Blochmann's figures (see Diagram 12, b), and this shows 

 reversals in quadrants B and D, so that the divisions are purely 

 bilateral; in Crepidula there is a reversal in quadrant D only, 

 so that the cleavage is bilateral in the posterior quadrants, but 

 not in the anterior ones. Umbrella shows almost exactly the 

 same reversals in the history of the third quartette as are 

 exhibited by Crepidula. All three of these gasteropods show 

 a slightly greater tendency to reversals in quadrants B and D 

 than in quadrants A and C. 



