40 bulletin: museum of COMPAP.ATIYE ZOOLOGY. 



Since the ruiuute cell d^' does not divide, we thus have produced 

 sixty-three cells instead of the typical number, sixty-four. Such a stage 

 does not, however, have an actual existence, since some of the divisions 

 belonging to the seventh cleavage have taken place before all these 

 cleavages are finished. 



Figure Gl (Plate 7) shows the anterior surface of the egg at the end 

 of the sixth cleavage, Figure 58 the posterior surface, Figure 60 the 

 dorsal pule, and Figure 63 (Plate 8) the ventral pole. 



Sequence of the Sixth Cleavage. — The order in which tlie cells divide 

 is, as I have already stated, now complicated by several factors. 



(1) The divisions of the first four quadrants of the egg (at the third 

 cleavage) were not synchronous, but followed in the order D, C, B, A. 

 Other conditions remaining the same, — that is, with equal intervals 

 between the ensuing cleavages, — the same order would obtain in the 

 later stages. 



(2) As discussed on page 35, the sequence becomes modified during 

 the fifth cleavage, so that the cells in any given quadrant divide in 

 nearly or quite the order of size of the blastomeres, beginning with the 

 largest. In the three quadrants A, B, and C, the order is (the ven- 

 tral cell in each case being considered number one) 4, 2, 1, 3, while in 

 the quadrant D the order is 1, 2, 4, 3. This order would naturally 

 reappear in the sixth cleavage, other conditions remaining the same. 



Both the above factors do influence the fifth cleavage, but with still 

 further complications. The first factor appears in the fact, that in any 

 given layer the general order of cleavage of the component cells is 

 D, C,B,A. 



The second factor is shown by considering the cleavage of a single 

 quadrant, as I). The order of cleavage for the large left hand cells of 

 this quadrant is as follows, naming the layers from ventral to dorsal : 

 1^ 2, \l, — nearly or quite the same as at the last cleavage. 



But a third factor appears in comparing the large left hand cells c?®-' 

 and d^-^ of the D quadrant with their small right hand sister cells d^'* 

 and d"-^ (Plate 5, Fig. 37, Plate 6, Fig. 46, Plate 7, Figs. 54 and 57). 

 T7ie large cells of each pair divide first, though the age of the two, being 

 Bister cells, is exactly the same. 



Similar relations may be shown for the other quadrants. Two facts 

 are worthy of particular notice. (1) The large cells a^-'-c'^-'' divide 

 first of all the cells in the quadrants A, B, and C, — and long before 

 the small cells a^-^-c^-^, which are of exactly the same age. (2) There 

 axe some variations which cannot be brought into relation with any of 



