396 



WILSON'. 



[Vol. VI. 



of c^ and d^ by the median division of X, as already described 

 (Figs. 31, 32). 



Shortly after the bilateral division of the posterior micro- 

 meres, c^ and d^, a similar bilateral unequal division takes place 

 in the anterior pair a)- and b^ (Figs. 35 to 38, Diagram II). As 

 this takes place, the micromeres move apart both from each 

 other and from the posterior micromeres so as to form an ex- 

 tremely regular and symmetrical cross, the centre of which is 

 occupied by the apical rosette. The three remaining intermedi- 

 ate girdle-cells {ci^^, b^-^, c^-"^) are at the same time drawn up into 

 the angles between the arms of the cross, and thus recede from 

 the girdle, though they do not leave it entirely until somewhat 

 later. This will be rendered clear by the accompanying diagram 

 (No. II). 



h_ ^ ^ t 



Diagram II. 

 The shaded cells represent the intermediate girdle-cells and their progeny. 



Nearly at the same time, the entire set of girdle-cells divide 

 in a very interesting symmetrical manner, which is illustrated 

 by the diagram and is shown in Figs. 35, '^6, 39. In Diagram 

 11, A {cf. Fig. 35) the direction of the division-spindles is shown 

 by the arrows. Diagram II, B {cf. Fig. 39), shows the result. 

 It will be seen that each intermediate girdle-cell divides in a 

 somewhat oblique plane, and is at the same time completely 

 drawn out of the girdle. The products of the trochoblasts 

 meanwhile divide in such wise that in each pair of cells one 

 divides horizontally and one vertically, in regular succession in 

 a left-handed spiral around the girdle. Thus each of the origi- 



