No. 3-] THE CELL-LINEAGE OF NEREIS. 379 



Others, is the "first somatoblast," and from it arises the ventral 

 plate. ^ 



The spiral character of the cleavage is strictly maintained in 

 the divisions of the micromeres which meanwhile take place. 

 At the close of the period (Fig. 29) the embryo consists of 

 thirty-eight cells, the relations of which to the germ-layers are 

 as follows : — 



4 Macromeres = Entoblast. 



,^. (. -x-x Ectomeres = Ectoblast. 



34 Micromeres \ "^ ^, , ^ , , , 



( I Mesomere = Mesoblast. 



Bilateral divisions now begin to appear among some of the 

 ectomeres, but others still continue to divide spirally ; i.e. the 

 bilateral period is overlapped to some extent by the spiral. 

 This period of overlapping, in which the number of cells in- 

 creases from thirty-eight to fifty-eight (Figs. 30 to 38), I shall 

 call the Transitional Period. Its principal event is the estab- 

 lishment of the prototrochal girdle and of a remarkable cross 

 of cells on the upper hemisphere which gives rise in part to the 

 cerebral ganglia. 



In the third, or Bilateral Period, the divisions become essen- 

 tially bilateral, and so remain as long as they can be followed. 

 The embryo is still perfectly spherical, and its cells are not 

 arranged according to a strict bilaterality, because they still 

 show traces of their spiral mode of origin. Little by little, how- 

 ever, a complete bilaterality is established, which becomes very 

 apparent as soon as the elongation of the body begins to take 

 place. 



The young trochophore (Fig. 82) is nearly spherical in form. 

 The prototroch occupies the equatorial plane, and is composed of 

 a girdle of twelve principal cells (derived from products of the 

 first set of micromeres). The girdle is interrupted in the 

 median posterior {i.e. dorsal) line by a narrow space through 

 which the small cells of the upper hemisphere are in continuity 

 with those of the lower. In the centre of the upper hemi- 

 sphere, at the point where the polar cells were formed, is an 

 apical tuft of cilia. Two eye-spots are symmetrically placed on 

 the upper hemisphere. The mouth, which opens into a large 



1 I shall adopt v. Wistinghausen's term " somatoblast " in place of the word 

 " proteloblast " employed in my earlier paper, reserving the latter term for a differ- 

 ent use (p. 407). 



