The Development of Isi'huocliiton. 595 



the second forms au inner set of four cells lying at the animal pole 

 — the apicals ; while the parent cells are spoken of in Gastropods as 

 the basal cells of the cross. As regards the basais, however, it will 

 be seeu that this terminology will not answer in the case of Ischio- 

 chiton, since at their next division they do not give rise to cross 

 cells, but to the accessory trochoblasts. After forming these 

 cells the parent cells in position and subsequent 

 history answer to the basal cells of the Molluscan cross. 



At about the time when the accessory trochoblasts are formed 

 the upper of the two cells of the second quartette in each quadrant 

 divides, forming the tip cells of the cross ('2a^-^^ etc.). The cross 

 in Ischnochiton therefore is composed at this stage of 12 cells, 4 

 apicals, 4 basais and 4 tip cells. In the 85 cell stage each basal 

 divides equatorially into an inner and outer cell, the basal proper 

 and the median cell of the cross (Fig. 17). The arms of the cross 

 are alike in each quadrant and taper from centre to tip, a condition 

 of affairs that is not disturbed by the subsequent radial division of 

 the basal cells. 



In the meantime the 4 apicals have divided into 8 cells, of which 

 4 lying symmetrically about the animal pole constitute the apical 

 rosette while the 4 peripheral cells are termed the peripheral rosette. 

 This rosette series also forms a cross whose arms lie midway between 

 the arms of the Molluscan cross. The tw'o crosses have hitherto been 

 considered together in Molluscs, but in Ischnochiton as the arms of 

 the Molluscan cross become dim and indistinct in outline the rosette 

 series comes into prominence as a distinct and beautifully symmetrical 

 cross whose arms lie wholly in the velar field. The history of these 

 two crosses is so widely different that it seems best to consider them 

 separately in their later stages. 



b) Axial Relations. The cross when formed is distinctly 

 dexiotropic, but this condition is somewhat modified by the formation 

 of the peripheral rosette, when the arms of the cross become more 

 radial in position. The tip cells of the cross however usually continue 

 to lie somewhat to the left of the basal cells, but when the primary 

 trochoblasts cleave the second time forming 4 cells in each quadrant 

 the tip cells assume a radial arrangement. This continues through 

 the cleavage of the basal and at least up to the time of the division 

 of the median cells. 



In the dexiotropic stage of the cross the long axis of the arms 

 does not coincide with the antero-posterior axis nor with the cor- 



