234 MEAD. [Vol. XIII. 



8-10). The sixteen cells (nearly ready to divide again) are 

 seen in Figs. 11-14. In consequence of the regularity of the 

 last cleavage, they are arranged in four alternating zones of four 

 cells each. The cells of the second zone on the upper hemisphere, 

 d'^, b^'^, f '■', ^'■', are the parent cells of the primary prototroch — 

 "trochoblasts" (Wilson), so I shall call them primary trocho- 

 blasts. In Amphitrite these cells are more strictly "trochoblasts " 

 than in Nereis, for all their descendants become prototrochal 

 cells, while in Nereis, according to Wilson, some do not. The 

 egg at this stage has a segmentation cavity of considerable size. 



As we shall see below, certain cells of the lower hemisphere, 

 a^, b", c^ also contribute to the prototroch in AmpJiitrite. 

 d {X), however, does not do so, but has a special destiny: it 

 is to form the whole ectoderm of the trunk, including procto- 

 doeal cells and paratroch. Therefore I shall call it, after Wilson, 

 the somatoblast. 



i6-J2 cells, Figs. 11-16. — All sixteen cells soon cleave 

 obliquely to the right, though not quite synchronously. The 

 two largest cells, d'^ and D^, divide first (Fig. 14); the cells of 

 the anterior hemisphere next (or frequently at the same time); 

 the remaining cells of the lower hemisphere last. Notwith- 

 standing these slight time variations, a 32-cell stage is at- 

 tained, which may be described as consisting of eight quartettes 

 of cells alternating with one another from pole to pole (Fig. 16). 

 The thirty-two cells are arranged as follows : a^.^, b^.^, c^.^, and d^.^ 

 meet at the animal pole ; in the outer angles between them lie 

 a^'^, b^'^, ^'■^ and ^'■^; alternating with these are four of the daughter 

 cells of the trochoblasts, a^'^'^-d^'^'^ ; and next the remaining four, 

 ^'■'■^-^'■'■^. These are all the cells of the anterior hemisphere . 

 Alternating with the last four cells, come d"'^, b'^'^, d'^, and d""^ (= 

 X), which, with the exception of ^°', contribute nearly all their 

 substance to the completion of the prototroch, and may be 

 called secondary trochoblasts . Next in order are the quartettes, 

 d'^-d^-", a^-d\ and at the lower pole, A^-D,. 



Up to this point, in the transition from one stage to another, 

 all the cells divide almost simultaneously and in the same 

 direction ; all the divisions are oblique, and the direction — 

 to the right or left — alternates with each succeeding stage 



