30 CELL-DIVISION AND GROWTH II. i 



fugalized with sufficient force the yolk is driven still more to- 

 wards the vegetative pole, while the protoplasm is accumulated in 

 the animal half of the egg. Such eggs segment meroblastically, 

 a cap of cells or blastoderm being formed lying on the surface 

 of a nucleated but undivided yolk. The yolk-nuclei, moreover, 

 are enlarged, as in megalecithal fish eggs (Fig. 15). 



The rule is, of course, only applicable to telolecithal eggs, and 

 for many of these it holds good, notably for Vertebrates. In 

 other classes there are, however, exceptions, which are best known 

 in those whose segmentation has been most carefully studied, 

 the ' spiral ' eggs of Turbellarians, Annelids, and Molluscs. 

 Large cells, in these ova, often divide more quickly than small 

 ones ; the second quartette of micromeres, for instance, is formed 

 before the first quartette divides in Crepidula, Unio, Limax, 

 TrocJius, Aplysia depilans, Diseocelis, and the cells of the third 

 quartette before the first products of division have had time 

 to divide again in Limax, Umbrella, and Aplysia limacina. 4 d is 

 often formed before the corresponding cells in the other quadrants 

 (in Unio, for example), but in Crepidula this is in accordance 

 with the rule, since 4 a, 4 b, and 4 c contain more yolk than 4 d. 

 In Arenicola, though the yolk is uniformly distributed, the cells 

 are still unequal. Other exceptions are to be found in the 

 continued unequal division of teloblasts, in the formation of the 

 micromeres in Echinoids, and in the unequal division of 

 the blastomeres in the third and fourth phases in Ctenophors. 

 According to Ziegler the formation of the micromeres in 

 Ctenophors cannot be due to the presence of yolk, since 

 they are still formed when part of the vegetative hemisphere 

 is removed, as Driesch and Morgan have also found. 



Ziegler indeed puts forward another hypothesis to account for 

 unequal division; he supposes that the centrosomes are hetero- 

 dynamic. So far there appears to be little evidence in support of 

 this view. It is quite true that in many cases of unequal division 

 the asters not the centrosomes vary in size with the size of the 

 cells. This occurs, for instance, in the division of the first micro- 

 meres and of the first somatoblast in Nereis, in the formation of 

 the first and second quartettes, and in the division of the first 

 somatoblast in Unio, in the division of the cell CD in Asplanchna. 



