60 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



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 meridio7ial, 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 b A, forming 6.7 and 6.8. 



Plate V. Figs. 35, 36 ; Plate 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 diff'erent 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 tlie division. 

 In Figure 36, h^* is the only one which has divided, resulting in h^'^ and 

 J^-^, 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 quadi'ants 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 ('94, p. 188), the divisions of this 

 generation begin before those of the sixth are completed. 



Division of Quartet 6.1, forming 7.1 a7id 7.2. 



Plate TI. Figs. 38, 40, 42. 



The division of this quartet is the point of greatest interest in the 

 cleavage, as it results in a difi'erentiation of the germ layers, or at least 

 in a separation of the primary mesoderm from the ect-entodei'm. 



