No. I.] THE EMBRYOLOGY OF CREPIDULA. 123 



occurred it is seen to be plainly laeotropic in 3a, 3b, and 3c 

 and dexiotropic^ in 3d, i.e., the cleavage is nearly bilateral on 

 the posterior side of the ovum ; it is not purely bilateral, be- 

 cause the lower division product, 3C2, lies nearer the mid line 

 on the right side than does the corresponding cell, 3d2, on the 

 left, Figs. 29-35. 



This is but another illustration of the fact that bilaterality 

 first appears on the posterior side of the Q^g, that it is due 

 to the change in direction of the cleavage of one out of four 

 cells, and that it is not perfect when it first appears, but is 

 merely a deviation from the spiral type toward the bilateral. 

 In this case also, as in every other, the oblique character of 

 the cleavage is much more pronounced after the daughter cells 

 are formed than during the nuclear division. 



By this first cleavage of the third quartette there is formed 

 an upper and a lower cell in each quadrant. The lower cell 

 (3a2, etc.) is a little smaller than the upper (3aS etc.). 



The next division of these cells occurs at the stage shown 

 in Figs. 36 and 38. The upper cells of this quartette on the 

 posterior side of the egg (30^ and 3di) divide before the others 

 and in a bilateral manner. Fig. 36. A little later. Fig. 38, the 

 corresponding cells on the anterior side of the egg {^^} and 

 3b') divide in a dexiotropic direction. There is now one lower 

 cell and a right (3a'-', etc.) and left (3a'% etc.) upper cell of 

 this quartette in each quadrant. 



Next the lower cell divides into right (3a^', etc.) and left 

 (3a^-^, etc.) halves in each quadrant. Fig. 43 and Diagram 7. 

 This division is slightly laeotropic in the anterior quadrants 

 but bilateral in the posterior ones. 



The outer products of this division on the posterior side of 

 the &gg (3c^' and 3d^') then divide bilaterally into upper and 

 lower products, 3c^-'-' and 3c^■'■^ 3d^-'' and 3d^''% Fig. 45. 



About the same time the outer upper cells in the posterior 

 quadrants (3c'-^ and 3d'-^) divide into upper and lower products, 

 (3c'-^' and 3c'-^^ 3d'-^-' and 3d'-^==) Figs. 44 and 45. 



1 The cleavage of this cell is not always reversed, for in some cases the division 

 of the nucleus may take place in the usual, i.e. laeotropic, direction, and the daugh- 

 ter nuclei may lie in this position relative to each other and yet the cell body may 

 show reversal of cleavage, e.g., 3d2 Fig. 35. 



