64 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the same in Figure 40, a*?, 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 twenty- 
nine-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, forming 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 /eft 
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 obscure all trace of its poles, and the absence 
"OO 
, 
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 mesoaerm 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 lineage 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 
