126 CONK LIN. [Vol. X 1 1 1 . 



his figures show, the spindles are placed radially. The lower 

 cell (3a2, etc.) in each case is smaller than the upper (3ai, etc.), 

 just as in Crepidula. After their formation, the lower deriva- 

 tives move in a laeotropic direction, and the cleavage might 

 therefore be considered as spirally dexiotropic. (The direction 

 of this cleavage is not clearly marked in Heymons' figures, 

 and, judging by his Figs. 8 and 12, it seems to me that the 

 cleavage of 3c and 3d might be considered laeotropic. If this 

 is really a dexiotropic cleavage, it is another violation of the 

 alternation of cleavages, since the preceding cleavage was 

 dexiotropic.) 



The next division of this quartette is peculiar, and closely 

 resembles the same cleavage in Crepidula. At the 38-cell 

 stage, the cells 3C1 and 3di divide before the cells 3ai and 3bi, 

 and in a bilateral manner. Exactly this same thing happens in 

 Crepidula. In Umbrella, the two products of this division 

 lying nearest the mid line are the " Exkretzellen," or proto- 

 blasts of the larval excretory organs, and are designated E and 

 Ei by Heymons. The cells formed by this division (3c'-' (= E) 

 and 3c'% 3d'' ( = Ei) and 3d'-^), are large and clear; they have 

 the same characteristics in Crepidula. 



The corresponding divisions in the anterior quadrants, A and 

 B, occur much later, viz., at a stage when there are 52 cells 

 present, and the cleavage is slightly laeotropic. In Crepidula, 

 this cleavage occurs soon after the divisions of the posterior 

 quadrants ; and the spindles are nearly horizontal, as they are in 

 Umbrella, but slightly dexiotropic instead of laeotropic. (From 

 the position of the two cell products after the division, as shown 

 in Heymons' Figs. 19 and 20, I should be inclined to consider 

 this cleavage as slightly laeotropic, almost bilateral, in Umbrella.) 



At the next cleavage there is a slight disagreement between 

 Umbrella and Crepidula. According to Heymons the two upper 

 cells in the posterior quadrant (3c'' (= E) and 3c'-% 3d'-' 

 (= Ei) and 3d'-^) divide in nearly a vertical direction, giving 

 rise to two small cells on each side of the mid line, which lie 

 alongside the lower cells 30^ and 3d2. A little later the cells 

 3a2 and 3b2 divide as they do in Crepidula, but 3C2 and 3d2 do 

 not divide. In Crepidula, on the other hand, all the lower 



