Experimental Studies on Germinal Localizaton. 205 



rically placed basal cells arises the ento-mesoblast, while as usual 

 the 28 remaining cells constitute the ectoblast. 



The sixth cleavages (32-64 cells) are in the main oblique and 

 leiotropic; but unlike Trochus the posterior micromeres of the 

 third quartet depart more or less widely from the type. Proceed- 

 ing from the upper pole downwards the divisions are as follows. 

 The rosette-cells (i/"^) divide nearly equally in regular spiral 

 order exactly as in Trochus^ so as to form a symmetrical group of 

 eight small cells at the upper pole (Fig. 7) which form, certainly 

 in part and probably as a whole, the basis of the apical organ. 

 Nearly at the same time the i^- cells divide nearly equally, so as 

 to form the primary "cross," which, as in Trochus, has at this 

 period spirally curved arms (Figs. 6, 7). The trochoblast-pairs 

 (i"-^ and I'-) divide equally, somewhat earlier than the fore- 

 going, so as to produce four symmetrically placed groups of four 

 equal cells (Figs. 5-7). This division takes place much earlier 

 than in Trochus, and no further division occurs In the products, 

 which become ciliated from the eighth to the tenth hour and form 

 the primary prototroch. The second quartet cells divide at about 

 the same time in a very characteristic fashion that is almost identi- 

 cal with that occuring In the nemertine egg and nearly similar to 

 that of Trochus. The upper left cell (2.^) divides slightly un- 

 equally, the smaller cell lying above and between the two ad- 

 joining trochoblast groups (Fig. 5). The lower right cell (2-) 

 divides still more unequally, the smaller lower cell (2--) lying 

 below against the corresponding macromere, and between the two 

 adjoining cells of the third quartet. In Trochus this cell Is smaller 

 still. The egg thus attains a 56-cell stage, at which a slight pause 

 occurs, and In the meantime a marked change occurs in one of the 

 macromeres which, I think. Is undoubtedly the posterior one, 3D. 

 This cell rapidly passes into the Interior, Its outer end becoming 

 greatly reduced, and being connected with a narrow neck with 

 a swollen interior portion, the nucleus however still lying at the 

 surface (Figs. 9, 13). The next cells to divide are those of the 

 third quartet. The two anterior ones divide leiotropically, like 

 the preceding micromeres. In the two posterior ones, however, 

 the spindles assume a bilateral position, with the central poles 



