No. 3.] THE CELL LINEAGE OF AMPHITRITE. 467 
Nevertheless, in Scolecolepis the eight cells divide syn- 
chronously and in the same direction as in Lepidonotus. 
Twelve of the sixteen resulting cells also divide together, and 
in the same direction as in Lepidonotus and Amphitrite. I 
have not observed the division of the other four. Since Sco- 
lecolepis has a suppressed trochophore, it is interesting that 
these and other cells, which in other forms give rise to the 
prototroch, are minute and slow to divide, The apical rosette 
is formed in the typical fashion. 
Germ-Layers. —In Amphitrite, at the sixty-four-cell stage, 
the germ-layers are segregated into definite cells. d+ (= J/) 
forms the mesoderm, a4, 54, ct, and A, B, C, D, form the ento- 
derm, the latter cells each dividing once before the invagina- 
tion. The rest of the cells form ectoderm. 
The mesoderm cells J/ and M, products of d@4, sink into the 
segmentation cavity and produce a pair of typical germ-bands 
by teloblastic budding. They do not bud off small cells upon 
the surface before they sink in, as has been found in Nereis, 
Unio, Umbrella, and other annelids and molluscs. However, 
when they have become elongated, and their nuclei have moved 
inward to about the middie of the cylindrical cell-body, each 
buds forth a small cell which is carried into the segmentation 
cavity with the large one and remains at the anterior end of 
the germ-band. The axes of the spindles in these divisions lie 
in the shortest diameter of the cells and apparently in the dtrec- 
tion of greatest pressure. 
Larval Organs.—In Amphitrite, the prototroch consists at 
first of sixteen cells, which are differentiated and become func- 
tional in the sixty-four-cell stage. They lie in four separate 
groups, of four cells each, and are descended from the “first 
generation of micromeres.” In Nereis, according to Wilson, 
only twelve of these sixteen cells are involved in the 
formation of the prototroch. Later the prototroch is com- 
pleted by the addition of nine more cells from the ‘second 
generation of micromeres,” in the quadrants A, 4, and C, 
respectively. No further cells are added in the Y quadrant, 
so an interruption still persists in this region — “dorsal 
interruption.” 
