THE DEVELOPMENT OF ORGANISMS 



733 



developing egg into prospective dorsal and ventral halves. But there 

 are many departures from this simple procedure. Thus when the 

 blastomeres are unequal in size, a spiral type of cleavage may result, 

 as in some marine worms and molluscs. There are interesting cases, 

 especially among insects and crustaceans, where many nuclear 

 divisions occur without there being divisions of the cytoplasm ; the 

 result is a "syncytium", but each nucleus eventually becomes the 

 centre of a defined cell. This suggests the conclusion elsewhere 



Fig. ii6. 



Different Modes of Segmentation shown by the Egg-Cells of Animals, i and 

 i', an ovum with almost no yolk shows total and equal segmentation 

 (e.g. starfish); 2 and 2', an ovum with a considerable amount of yolk 

 shows total and unequal segmentation (e.g. frog); 3 and 3', an ovum with 

 much yolk at one pole shows partial and discoidal segmentation (e.g. 

 bird) ; 4 and 4', an ovum with a central core of abundant yolk show's 

 partial and peripheral segmentation (e.g. insect). N, nucleus; y, yolk; 

 ys, yolk-laden cells; hi, blastoderm or segmented disc. 



referred to that the segregation of cells is an arrangement which 

 facilitates division of labour or differentiation. 



In many cases, such as sea-urchin and frog, where the ovum is 

 radially symmetrical to begin with, the first cleavage establishes 

 bilateral symmetry, dividing the egg into prospective right and left 

 halves. But the eggs of squids and of many insects have a bilateral 

 shape and structure before fertilisation, and the axes of symmetry 

 of the egg are the same as those of the embryo. It is evident, then, 

 that there is considerable variety in segmentation. The four main 

 types have been already described (see Figs. 116 and 117). 



Early Stages. — The result of the cleavage or segmentation of 



