No. I.] THE EMBRYOLOGY OF CREPIDULA. 115 



forty-four cells of this quartette are present ; this has been due 

 both to the great number of cells in the entire egg and to the 

 lack of landmarks to which I have already referred. 



The first division of the cells of the second quartette has 

 been described (p. 63) ; by it each cell is equally divided in a 

 dexiotropic direction into right and left halves (2ai and 2a2, etc.), 

 Fig. 18 and Diagram 4. 



At the second division, which was described on p. Z^, the 

 right half is unequally divided, in a laeotropic direction, into a 

 small upper and a large lower cell (2a'' and 2a'% etc.). Figs. 

 26-28 and Diagram 6. The upper cell in each quadrant forms 

 the terminal cell in one of the arms of the cross. At the same 

 time. Figs. 26-28 and Diagram 6, the left half (2a2, etc.) divides 

 into an upper and a lower cell (2a^' and 2a2^ etc.) by a cleavage 

 which is slightly laeotropic, almost radial. Of these two cells 

 the upper one is slightly the larger. There are now four cells 

 of this quartette in each quadrant, a right upper and lower (the 

 right upper is the tip cell) and a left upper and lower. Diagram 

 10 and Figs. 29-33. 



Next the right lower and left upper (2a'-^ and 2a^', etc.) 

 divide simultaneously in each quadrant, though in the posterior 

 quadrant the division is later than in the other three, Figs. 35, 

 38, 39, and Diagram 7. In each case the direction of the 

 cleavage is slightly laeotropic in the right cell and dexiotropic 

 in the left. The previous division of the right half (to form the 

 tip cells) was laeotropic, so that here we have another violation 

 of the law of alternating cleavages. By this division six cells 

 in each quadrant are formed, — an upper, middle, and lower right, 

 and an upper, middle, and lower left. Diagram 7 and Figs. 40, 



43- 



The first of these six cells to divide is the upper right or the 

 tip cells in the arms of the cross. The tip cell of the posterior 

 arm divides before the others. Fig. 42, in a slightly dexiotropic 

 direction. The other tip cells divide a little later, Figs. 44, 45, 

 in a direction which is more or less variable, being usually, 

 however, dexiotropic in the left and anterior arms and laeo- 

 tropic in the right. Inasmuch as all these cells were formed 

 by laeotropic cleavage, the subsequent division of one of them 



