Experimental Studies on Germinal Localization. 219 



are grouped in small tufts (an arrangement I failed to note in 

 the isolated cells) at the base of each of which is a very distinct 

 deeply staining basal body; and I believe this would be an ex- 

 cellent object for the cytological study of the possible relation of 

 these bodies to the centrosome. 



3. Development of the sister-cell (i^) of the primary trocho- 

 blast ( i/i6-embryos) . 



The development of this cell, which, except for its slightly 

 smaller size, is indistinguishable in appearance from the primary 

 trochoblast, differs totally from the foregoing in the form and 

 rhythm of cleavage, and in the course of its differentiation. After 

 isolation this cell typically divides unequally to form a single 

 rosette cell (i^-^) (though here, too, the inequality is often less 

 marked than in the normal embryo, and sometimes disappears), 

 and its larger sister (i^"), (Figs. 58-60) ; and this is typically fol- 

 lowed by a nearly equal division of both these cells to form a 

 group of four, two of which obviously represent daughter-rosette 

 cells (Figs. 61, 62). The divisions do not cease here, however, 

 but continue, and at the end of 24 hours a larva is produced that 

 consists of many cells and is somewhat similar to those arising 

 from an entire micromere of the first quartet; this larva is, how- 

 ever, only about half as large, and lacks the four large trocho- 

 blasts at the posterior end (Figs. 63-64). At this end of the 

 embryo are two trochoblasts, much smaller than those of the pri- 

 mary group, which obviously represent the secondary trocho- 

 blasts of the first quartet. At the narrower anterior end is an 

 apical organ precisely like that of the micromere j/^-embryo, 

 while the middle region consists of small ectoblast cells usually 

 larger on one side than on the other. These embryos swim ac- 

 tively, but less vigorously than the J^-forms, and, like the lat- 

 ter, perish in the course of the second or third day. It is obvious 

 that the development of the i^ cell is, except for its closure, es- 

 sentially the same when isolated as when it forms part of a whole 

 embryo; and its remarkable contrast with its sister-cell, the pri- 

 mary trochoblast, shows most convincingly that despite their 



