396 GEORGE W. TANNREUTHER 



belongs. Thus aS b^ -^ c'-i-, or d^-^^ all belong to the first genera- 

 tion; c^, b^-^ or d^-^, belong to the second generation, and a^S 

 133.2.2^ c*-2, or d^-^, etc., belong to the third generation. 



On account of the peculiar shifting of the macromeres A, B, 

 C, and D in the formation of the first quartette. A, B, and C 

 take a position more anterior and D posterior, instead of at the 

 vegetal pole, as in annelids. When a cell divides, the product 

 receives the designation of the parent cell with the addition of a 



f C2-l 



further index number; thus, c^^ „ . The cell D, after the for- 



mation of d^ and d^, is designated by the capital letter E ; it gives 

 rise to the reproductive system and all of the endoderm (digestive 

 system) except the stomodaeum and the pharynx. 



2. Nature of cleavage 



First cleavage: Immediately after maturation the nucleus 

 passes from the surface toward the center of the egg, but nearer 

 the anterior end. The first cleavage spindle is formed about 

 thirty minutes after maturation (figs. 13 to 16), the time varying 

 somewhat with external conditions. Low temperature retards 

 the rate of cleavage. The first cleavage spindle occurs in the 

 plane of the long axis of the egg (fig. 7). It passes through the 

 region of the polar body or bodies and divides the egg into two 

 very unequal parts, AB and CD (figs. 15 to 17). The smaller 

 cell, AB, is anterior and the larger cell, CD, is posterior. The 

 cleavage furrow at first is deep and the cells are rounded, but 

 before the second cleavage occurs the cells flatten at their point 

 of contact and the egg becomes more elliptical with the first 

 cleavage plane scarcely visible. 



The granular content of the cells is uniform, with very few 

 yolk bodies visible. The region immediately surrounding the 

 nucleus is almost free from cytoplasmic granules and makes it 

 possible to follow the nuclear activities in the process of division 

 in the living egg. The first cleavage in the female- and the male- 

 producing egg is the same. It occurs at right angles to the future 

 longitudinal axis of the adult. The second cleavage plane occurs 



