584 HAROLD HEATH, 



The fuller significance of this cleavage will be considered in the 

 section on the Molluscan cross, though it may be added in this con- 

 nection that the parent cells of the accessory trochoblasts are now to 

 be considered as the basal cells of the cross, since in their future 

 history they bear a remarkable likeness to the cross cells in the 

 Gastropod. 



5. Formation of Tertiary Stomatoblasts; Cleavage of 

 A pic a Is; Origin of Secondary Trochoblasts (Tip Cells); 

 Division of Basais; Second Cleavage of Primary Trocho- 

 blasts; Formation of left Post-Trochal Cells, 63 cells 



(Figs. 16, 17). 



About this time a leiotropic division occurs in each cell of the 

 third quartette by which four cells are formed about one third the 

 size of the parent cell (Fig. 16). Immediately after their formation 

 they show most clearly their leiotropic position , crowding onto the 

 territory of the second quartette and macromere of the next quadrant, 

 but very rapidly they adjust themselves symmetrically in the angles 

 between the macromeres (Figs. 20, 22), These cells form by far the 

 greater part of the stomodaeura, and following Wilson I shall term 

 them stomatoblasts. This designation in the case of Nereis however 

 was applied to second quartette cells which in Ischnochiton also enter 

 the stomodaeum, and in view of this fact I have designated both sets 

 of cells stomatoblasts, speaking of them either as second or third 

 quartette stomatoblasts as the case may be. All these third quartette 

 stomatoblasts appear of about the same size, although later it will be 

 noted that the size of the two posterior exceeds that of the anterior, 

 which aids very materially in bringing about marked changes in the 

 form of the embryo. 



The 44 cell stage is ushered in by the division of the apicals 

 (Fig. 14). In some cases the spindles are perfectly meridional, but 

 the division is invariably leiotropic. The area of the "rosette series" 

 is thus increased but its relative position remains the same and this 

 is true as far as its history has been traced, the tips of its arms 

 always maintaining the same axial relations with the accessory 

 trochoblasts. 



The next division affects the upper cell of the second quartette 

 (Fig. 16). By a leiotropic cleavage it divides into a triangular cell 

 lying to the left and somewhat above the remaining narrow rect- 

 angular one. The upper cell is the tip cell of the cross, and is a 



