1 62 GARDINER. [Vol. xi. 



in shape at times when the karyokinetic activity is at its 

 highest as compared to that when the ovum is in its resting 

 stage. The ovum shown in Fig. lo soon rounds up so as to 

 be indistinguishable from that shown in Fig. 12. Not infre- 

 quently, however, one part will round up some time before 

 the other, suggesting most strongly the absence of the antero- 

 posterior and bilateral symmetry which characterizes this form 

 of segmentation. 



To turn now to the segmentation of the ovum. Shortly 

 after the ova are laid, the pigment granules form a complete 

 girdle round the smaller axis of each ovum (Fig. i) and mass 

 themselves also in the region where the cells {C, C) will be 

 formed, and then the ovum divides into two cells of equal size 

 {A and A). The second cleavage plane is in the region of this 

 pigmented area. It is at right angles to the first and divides 

 the cells A, A', into A, B, and A', B' (Fig. 2). Immediately 

 after these cells (B and B') have been budded off, they begin 

 to rotate from left to right, so that B, instead of lying directly 

 above A, and B' directly above A', both B and B' cover the 

 line of the first cleavage plane and rest on both A and A' 

 (Fig. 3). Fig. 4 shows the position attained by B as seen 

 from the side, while B' passes to the opposite side of the ovum. 

 After this four-celled stage has been reached a period of rest 

 follows, the ovum becomes again oval, and all cell outline dis- 

 appears. If it were not for the bright girdle of pigment and 

 the massing of the granules in the neighborhood of B and B', the 

 ovum might easily be mistaken for one just laid and unsegmented. 



The appearance of the third cleavage plane is preceded by 

 the reappearance of the cell outline and also by a massing of 

 the pigment granules on the surface of A, A' in the neighbor- 

 hood of B and B', which cells move apart to right or left so 

 that the line of the first cleavage plane may be seen between 

 them. At the same time the cells C and C are budded off 

 from A and A' (Fig. 5), exactly as were B and B'. No rota- 

 tion, however, takes place, but the cells C and C round up and 

 draw closer together in such a manner that B and B' are forced 

 still farther apart and come finally to lie on opposite sides of 

 the ovum, as is shown in Figs. 6, 7, 8 and 1 1. j 



