48 



bulletin: museum of comparative zoology. 



The cleavage of rf'-" occurs much later, and is likewise equatorial. 

 I have not thought it necessary to introduce a special figure to show the 

 cleavage of this minute cell. 



In (f '■^* the cleavage also takes place late; it is meridional and equal. 

 The resulting cells, d'-"^ and c?^-^, are shown in Figure 72 (Plate 9). 

 The minute dorsal cell tf -^^ does not divide. 



L R 



Diagram IV. 



Quadrant D in the eifrhth genera- 

 tion, — except the dorsal cell, (P-^^, 

 which does not divide farther. In 

 the other cells only the second ex- 

 ponent is expressed, the first being 

 in all cases 8. The arrows connect 

 cells of common orisin. and show the 

 direction of the spindles at the pre- 

 ceding^ cleava^re. 



Rsipnifies right; L, left; D, dorsal; 

 V, ventral. 



Diagram V. 

 Diagram of quadrant A, B, or C, in 

 the eighth generation. Only the second 

 exponent of the cells appears in the dia- 

 gram, the first being 8. The small dor- 

 sal cell, (a. b. or c)^'i6, does not divide, 

 remaining thus in the seventh genera- 

 tion. The arrows connect cells of com- 

 mon origin, and show the direction of 

 the spindle at the preceding division. 



R signifies right; L, left; D, dorsal; 

 V, ventral. 



There are thus 23 cells in the ectodermal part of the quadrant D at 

 the end of the seventh cleavage. A diagram of this stage is annexed 

 (Diagram IV.). Nearly this stage is represented in Figure 68. 



(2) Tlie Quadrants A, B, and C. — Owing to the regularity in the 

 size of the cells in these quadrants, and the fact that they are purely 

 ectodermal, the conditions observed are fairly simple as compared with 



