BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
60 
and opacity of this egg the details in the periphery, i. e. the region of 
the cells in question, are very much obscured, and I consider this deter- 
mination questionable. 
The comparison of the division of this quartet with the corresponding 
one in Nereis is very interesting. In Nereis, as in Limax, the division 
is nearly meridional, and with traces of a right spiral. In this instance 
it takes place at the twenty-nine-cell stage, and the products form the 
prototroch. In Umbrella it does not take place till the sixty-five-cell 
stage, and here also exhibits an obscure right spiral. (See Tables of 
Cleavage, pp. 66, 74, and 75.) 
Division of Quartet 5,4, forming 6.7 and 6.8. 
Plate V. Figs. 35, 36; Plato VI. Figs. 37, 39, 41. 
This is one of the first divisions to follow the twenty-four-cell stage. 
Figures 35 and 36 (Plate V.) show it in progress; Figures 37, 39, and 41 
after completion, It takes place in a very evident right spiral, the upper 
aster and derivative lying to the right in every case. The division is 
approximately equal, but is not synchronous in the different quadrants, 
as is shown in Figures 35 and 36. In Figure 35 all the cells of this 
quartet have divided except a’*; the quadrant c, judging from the size 
and position of the daughter nuclei, has evidently led in the division. 
In Figure 36, 0^* is the only one which has divided, resulting in 09" and 
0^9, the other cells containing spindles. It is evident from these two cases 
that it is impossible to predicate any regular sequence in the successive 
divisions of the quadrants of this quartet. 
This completes the discussion of the cleavages of this generation. It 
will be noted that all of the divisions clearly take place in a right spiral, 
with the exception of that of 5.3, and that this, though predominantly 
meridional, still shows traces of a right spiral. 
SEVENTH GENERATION. SIXTY-FOUR CELLS. 
As was stated in my earlier paper (794, p. 188), the divisions of this 
generation begin before those of the sixth are completed. 
Division of Quartet 6.1, forming 7.1 and 7.2. 
Plate VI, Figs, 38, 40, 42. 
The division of this quartet is the point of greatest interest in the 
cleavage, as it results in a differentiation of the germ layers, or at least 
in a separation of the primary mesoderm from the ect-entoderm. 
