86 CONKLIN. [Vol. XIII. 



which is continued until, as shown in Fig. 49, they are split 

 from base to tip. 



Before this longitudinal division of these three arms is com- 

 pleted, the four central or apical cells divide in a laeotropic 

 direction ; by this division four central and four peripheral 

 cells are formed. The former (la'-'-'-id'-'') are the apical rosettes 

 (Wilson ('92), p. 392) ; the latter are 'Cuo, peripheral rosettes {is}-^-^- 

 id'-'-^). The peripheral rosettes are slightly larger than the 

 apical cells, and lie just central to the turret cells and between 

 the basal cells of adjacent arms. Figs. 44, 45. The division 

 of the four apical cells (la'-'-id'-') is rarely simultaneous, and 

 yet the sequence of cleavage follows no invariable order. In 

 the ova figured the cells of the second and fourth quadrant have 

 divided, while those of the first and third are just dividing. 



At the same time that the apical cells are dividing, the ter- 

 minal cell of each arm, except the posterior, divides into two 

 small cells. This division is frequently very irregular ; in 

 Figs. 44 and 45 it is dexiotropic in the right and anterior arms, 

 and laeotropic in the left ; in other words, the cleavage is bilat- 

 eral in the transverse arms. This is, I think, the most frequent 

 condition, but there are many deviations from this form. The 

 products of this division are the right and left tip cells. Finally, 

 the longitudinal splitting of all the arms, except the posterior, 

 is completed by the equal division of the basal cells (Figs. 46, 

 47, and Diagram 8) into right and left portions, the right and 

 left basals (la'-^'-' and ia'-^-'-% ib'-^'' and ib'-^-'-S etc.). This 

 division is very nearly meridional, but subsequent stages, e.g., 

 Fig. 50, show that the left moiety is a little nearer the apical 

 pole than the right ; the division is, therefore, laeotropic. 



The cross now consists of 30 cells, as follows : 



Apical cells 4 



Peripheral rosettes 4 



Post, arm — i basal, i middle, 2 terminal 4 



Ant. right and left arms, each 2 basal, 2 middle, 2 terminal cells, 6 . 



in each arm. In 3 arms 18 



Total 30 



These cells all belong to the first quartette except the two 

 terminal cells of each arm, which were derived from the second 

 quartette. Diagram 8. The cells in three of the arms, the ante- 



