64 BULLETIN : MUSEUM OF COMPARATIVE ZO*dJiOGY. 



the same in Figure 40, c^}^^, a^*. In all the other figures the division 

 seems to be equatorial. A comparison with [Nereis reveals in this 

 instance the same marked agreement noted for the meridional cleavage 

 of the quartet 5.3. In Umbrella this division takes place at the twentj- 

 niue-cell stage, and is also equatorial. 



Division of Quartet 6.3, forming 7.5 and 7.6. 



Plate VI. Figs. 41, 42. 



This division is in progress in the quadrant c in Figures 41, 42, and 

 the other members of the quartet are also approaching mitosis. There 

 is a faint trace of a left spiral to be detected in the position of c^^^ and 

 c'^^ of Figure 41, but the division is predominantly equatorial. 



Division of Quartet 6.4:, forminc/ 7.7 and 7.8. 



Plate VI. Figs. 39, 40, 41, 42. 



In the two eggs figured the spiral is clearly shown by the relative 

 position of the nuclei to be a left spiral. Thus all the spirals of this 

 seventh generation, wherever they have been traced, have been left 

 spirals. 



With this forty-five-cell stage my detailed account of the cleavage 

 closes. I have not been able to decipher satisfactorily the conditions in 

 the eggs of the next stage, because during this stage a large number of 

 cells divide, — in one instance as many as thirteen. Moreover, the 

 rounded contours of the mitotic cells produce such changes in the surface 

 of the egg as effectually to oltscure all trace of its poles, and the absence 

 of polar globules, of macromeres, or of any " landmarks " whatever for 

 orientation, makes any interpretation of these later stages at the best 

 provisional, and very largely conjectural. Added to these difficulties is 

 that produced by the vacuolation which prevailed in a very large pro- 

 portion of the eggs which I have examined. This distorts and obscures 

 the relation of cells to such an extent as to make a determination of 

 their lineage extremely difficult, if not impossible. 



As late, however, as the hundred-cell stage, when four mesoderm cells 

 are present, it is possible on favorable eggs to work out a provisional 

 lineage, but I have not as yet succeeded in connecting this with the 

 forty-five-cell stage. 



Thus the outcome of my work as a study in cell linear/e is a disappoint- 

 ment, for I have not been able to trace a single blastomere to a definite 

 organ of the adult. At the stage of thirty-eight cells in Nereis, Wilson 



i 



