582 WILSON. [Vol. VIII. 



the formation of four smaller cells (a'^-d,'^ first group of 

 micromeres), lying exactly above four larger cells, which I 

 shall call the primary macromeres (A-D, Figs. 6, 7). This 

 agrees precisely with Hatschek's description and with the 

 cleavage of Antedo7i as described by Seeliger (28). Cross- 

 furrows may or may not be present, but in the former case 

 are inconstant in position. In Fig. 7 they are equal and 

 parallel. This form of division occurred in at least three- 

 fourths of all the eggs observed. 



b. In the spiral form (Figs. 8, 9, 10) the spindles are 

 obliquely placed so that the upper quaterfoil of cells is rotated 

 upon the lower, and invariably in the same direction, with 

 the hands of a watch (right-handed spiral in Lang's sense), 

 as in annelids, mollusks, and polyclades. All degrees of rota- 

 tion exist from 45° (strict alternation of the blastomeres) down 

 to a scarcely noticeable displacement. Cross-furrows almost 

 invariably present. This form of division occurred in about 

 one-fifth of the eggs observed. 



c. The bilateral form (Figs. 12 and 13) differs at this stage 

 from the radial only in the fact that two (sometimes all four) 

 of the macromeres become slightly separated from each other 

 along the line of the first cleavage. In the 8-celled stage, 

 therefore, the embryo can be divided into corresponding halves 

 only in the plane of the first cleavage. This form is rare in 

 the 8-celled stage, though very common in the succeeding 

 stage. I have followed it in the living egg in only a single 

 case. (Figs. 13 to 18.) In this individual the gap between 

 the two lower cells first arose during the third cleavage. 



Various transitional forms exist between the three types 

 described. Fig. 1 1 represents such a form, viewed from the 

 lower pole. The four lower cells are distinctly bilateral in 

 arrangement, but the upper quaterfoil is rotated nearly 45 

 degrees upon the lower. 



3. The fourth cleavage exhibits an interesting series of 

 forms, which may be arranged under the same three types 

 as those of the third, with numerous transitional forms. 



a. Radial form. This was the only form observed by 

 Hatschek, whose description is as follows: "Es zerfallt jede 



