534 PROCEEDINGS OF THE ACADEMY OF [DeC, 



small cell is cut off at the lower edge (figs. 24, 27). The same is true of 

 the divisions of la^-^-lcPK On the other hand the cells la^-^-ld}-^ 

 do not show such an unequal division. These latter cells have been 

 traced accurately through only three or four divisions, but the resulting 

 cells are all more or less equal in size and apparently wall enter directly 

 into the formation of the ectoderm. This latter process, i.e., the equal 

 division of the ectoderm cells, is much more in accord with what haii 

 been found in annelids and mollusks than is the former. 



History of the Second and Third Quartets. 



Just after the formation of the apical cells, and at about the time the 

 large cells SA and 3C are dividing, the larger cells of the second quartet, 

 2a^-2(P, divide in a nearly vertical direction (fig. 17). In many cases^ 

 the spindles show a slight dexiotropic tiu'n. Since the resulting distal 

 cells 2a'^-^-2(P-^ come to lie in the furrows between the large cells of the 

 fourth quartet, they are somewhat shifted in position to accommodate 

 themselves to the inequalities of these large cells. 



It thus happens that after the cells are separated some may be to the 

 right of the upper cell and others to their left. The division is always 

 nearly radial, but is still to be considered as belonging to the series of 

 spiral divisions. 



With this w^e have traced the divisions of the second quartet until 

 there are twelve cells present, three in each quadrant and all on the 

 surface of the egg. 



Just after the mesentoblast cell has divided for the first time the cells 

 2a^-2d} divide almost vertically but in a slightly Iseotropic direction. 

 The lower or distal cells, 2a^-^-2d'^'^, are very much smaller than the 

 upper ones (fig. 21). The former lie on the surface of the latter. This 

 is another example of the cutting off of a small cell to the exterior. 

 At about this time, sometimes before and sometimes afterwards, the 

 small cells 2a?-'^-2<p-'^, lying in the furrows between the four primary 

 cells of the fourth quartet, divide. The direction of the nuclear spindle 

 is nearly vertical (figs. 26, 27). The two cells seem nearly equal in size. 

 The lower cells, 2a^-'^-^-2d?-'^-^, reach almost to the upper edges of the 

 macromeres (figs. 26, 30). The descendants of these cells are the first 

 to reach the lower pole of the egg, and form Avithout a doubt a consider- 

 able portion of the ectodermal pharynx. 



Shortly after these two divisions the large cells of the second quartet,. 

 2a?-^-2d?\ divide in a dexiotropic manner, the two cells varying only 

 slightly in size (figs. 25, 27). At this time there are twenty-four cells 

 in the second quartet, six in each quadrant, and about seventy-eight 



