696 PROCEEDINGS OF THE ACADEMY OF [Oct., 



is D, C, B, A. This order of division is maintained by the macromeres 

 through five successive cleavages, and is more or less perfectlj^ retained 

 by their descendants. This succession is well shown in the ovum 

 represented in fig. 8, where the nuclei of the cells ID, IC, Id and Ic 

 are in the prophase of the next division, while a bundle of spindle 

 fibres still connects the nuclei of lA and la and the micromere 11) is 

 scarcely yet cut off from IB. This sequence in the division of the 

 macromeres is clearly correlated with the difference in the sizes of A 

 B, C and D, since it has already been pointed out that in relative size 

 the macromeres follow one another in the order D, C, A, B, which is 

 also the order of their division. It is evident that this is only a special 

 instance of a widespread class of phenomena, discussed by Kofoid 

 (1894), who points out that in many ova the larger blastomeres, i.e., 

 those containing a greater amount of yolk, or rather more cytoplasm, 

 tend to divide more rapidly than do the smaller ones. This, as Kofoid 

 shows, is in contradiction to Balfour (1880), who formulated a law 

 which supposes yolk to retard the cleavage. Kofoid ingeniously 

 explains the contradiction l^y suggesting that "difference in the rapid- 

 ity of cleavage is apparently correlated with the greater or less absolute 

 amount of protoplasm," and that the amount of protoplasm in turn 

 may be increased through the appropriation of yolk and the ratio of 

 division may be thus indirectly hastened. In Dinophilus, however, 

 while it seems tolerably plain that the difference in the time of division 

 of the macromeres is related to their difference in size, yet the latter 

 fact at first sight appears scarcely adequate to explain the great delay 

 in the divisions of B as compared with those of A, since between these 

 two blastomeres the difference in size is comparatively slight. On 

 the other hand, it must be remembered that since the macromeres 

 have different rates of division the gap between the time of division of 

 the four blastomeres may be considerably widened as the cleavage 

 progresses; in other words, A may start with but a slight lead over 

 B, yet it is, through its more rapid rate of division, enabled to increase 

 that lead in subsequent divisions. This difference in the rates of 

 division of the four quadrants will become obvious by reference to 

 the table of cell-lineage at the back of the i)aper. 



The spindles for the third cleavage are inclined strongly to the right, 

 and the daughter cells, the first quartette of ectomeres, when first 

 formed lie in the furrow between the macromeres. The diameter of 

 these cells, as figs. 8 and 9 show, is about two-thirds of that of the 

 macromeres from which they arose. They are not, however, all of the 

 same size, the two posterior micromeres Id and Ic being equal in size 



