204 Edmund B. Wilson. 



from the fact that in the early stages it often shows no cross- 

 furrow (thus differing from Trochiis) ^ or if one is present it is 

 very short (Fig. i), though in the 32-cell stage a characteristic 

 cross-furrow is present at the lower pole (Fig. 13). It is there- 

 fore impossible to identify the quadrants in the earlier stages 

 without having observed the divisions from the beginning. As 

 usual, three quartets of ectomeres are successively formed by al- 

 ternating dexiotropic and leiotropic spiral or oblique divisions. 

 The micromeres of the first quartet, often only slightly displaced 

 towards the left, are considerably smaller than the basal cells, 

 but are relatively larger than in Trochiis (Fig. 2). The fourth 

 cleavage is closely similar to that of Trochiis^ each of the upper 

 cells dividing slightly unequally and each of the basals somewhat 

 more unequally to form the second quartet. In the i6-cell stage 

 (Fig. 3) the egg consists as usual of four large basal cells (2A, 

 2B, 2C, 2D), four smaller upper cells (la^ — id^) and eight 

 alternating cells surrounding the equatorial region. Four of 

 these, of equal size, form the second quartet (2a — 2d). The al- 

 ternating four, which are somewhat smaller (la- — id"), are the 

 primary trochoblasts,^ by two successive equal divisions of which 

 arise the sixteen cells of the primary prototroch. The i6-cell stage 

 is thus closely similar to that of Trochus, except that the basal 

 cells are relatively smaller while all the others are relatively 

 larger. 



The fifth cleavages are dexiotropic and symmetrical through- 

 out the embryo, and again agree in the main with those of Tro- 

 chus. Each of the basals divides unequally to form a cell of the 

 third quartet, relatively somewhat larger than in Trochiis^ while 

 each cell of the second quartet divides nearly equally (in Trochus 

 this division is distinctly unequal). The primary trochoblasts di- 

 vide equally, the upper cells unequally, so as to form at the upper 

 pole a rosette of smaller cells (Fig. 6) almost identical with those 

 in Trochus, but slightly larger. 



The 32-cell stage thus attained (Fig. 4) is at first perfectly 

 radially (spirally) symmetrical. From the four large symmet- 



^These cells, and their products, are stippled in all of the figures 



