REPRODUCTION AND DEVELOPMENT 



117 



chaetes and oligochaetes. While there is great individual variation 

 in both the method of cleavage and the type of larva formed, it is 

 possible to discern a general pattern of development which runs 

 somewhat as follows. Two divisions in a vertical plane separate 



Fig. 68. Diagram illustrating spiral cleavage. From Waddington, 



1956. 



the fertilized egg into four blastomeres, usually of unequal size. 

 These are conventionally known as the macromeres A, B, C and D. 

 Each macromere then cuts off a small cell, a micromere, at one end 

 of the egg (the animal pole) and these are named la, lb, Ic and Id, 

 Instead of lying immediately above the macromeres which gave 

 rise to them the micromeres lie a little to one side owing to tilting 

 of the cleavage spindle during division. A second set of micro- 

 meres 2<2, 2b, 2c and 2d are now formed but when the first set are 

 displaced clockwise the second set are displaced anticlockwise, and 

 vice versa. A third set is formed and a fourth, each being offset in 

 the opposite direction from its predecessor. While this is going on 

 the earlier sets of micromeres themselves divide, still with their 

 spindles tilted. In this way a hollow blastula stage is formed with 

 a cap of micromeres surmounting four macromeres. If the latter 



