No. I.] POLYCHOERUS CAUDATUS. 167 



thirteen derivatives from E ; in VI the twelve derivatives from 

 Z>'; in VII the thirteen derivatives from E \ maldng in all 

 sixty-six cells, all, except A, A', which are much the largest, of 

 about the same size. 



Thus we have, as it were, two very different methods of 

 segmentation shown in different periods during the formation of 

 the sixty-six-celled stage. At first four pairs of cells, B, B\ C, C, 

 D, D\ and E, E', were budded off from the two macromeres A 

 and A' in successive generations. From now on segmentation 

 progresses by the division of these eight cells, while the 

 remnants of the two macromeres undergo no further division 

 till much later. Further, the first four generations follow 

 always in the order in which they are named {B and B', C and 

 C, D and B', E and E') but from now on the generations are 

 less easy to distinguish. For instance, in some cases D and 

 B' give rise to Br, Bl, B'r and B'l before the cells E and E' 

 divide at all, while in other cases E and E' give rise respectively 

 to El, E2, -£"3, E'l, E'2, E' 2,, at the same time or even before 

 B and B' divide. Hence it is impossible to distinguish with 

 accuracy the relative ages of the derivatives of the eight cells. 

 They appear, however, to be formed most generally in the order 

 above related, which is also shown in the diagrammatic cell 

 lineage shown on p. 168. From the appearance of the first 

 cleavage plane on, both bilateral and antero-posterior symmetry 

 are maintained. By subjecting the ten-celled stage ova to 

 pressure by crushing them slowly, and also by sections, a better 

 idea of the relative size of the cells can be obtained than by 

 merely surface views, and it is apparent that C and C are 

 the smallest of those budded from A, A' ; B, B' are next larger 

 in size, B, B' next, and E, E' the largest. 



To form the sixty-six cells 



C, C have given rise to 4 cells 

 B, B' " " " 10 " 



D, D' " " " 24 " 



E, E' " " " 26 " 



5 The remnants of the ") ^4 cells of about the samesize (Fig. 22). 



i two first macromeres \ ^' ^' " " ^ ^^^^^ ^''^''g^'' ^^an the above. 



66 cells. 



