No. 3.] THE CELE LINEAGE OF AMPHITRITE. 469 
At the sixty-four-cell stage we can first speak of the trocho- 
phore. 
At this time the germ-layers are segregated into definite 
cells, and the primary prototroch is functional, so that the 
animal swims about. 
The trochophore is now spheroidal. The rosette cells, with 
polar globules still attached, mark the anterior end, while 
the posterior end lies nearly 180° away at the fosterior 
end of the somatic plate, t.e., the cells of the d? or X group 
(ventral plate, Wilson). 
The primary prototroch, and the cells which complete the 
prototroch, constitute a transverse band about the trochophore, 
separating the anterior from the posterior hemispheres, except 
at the interruption in the mid-dorsal region. 
The form of the anterior hemisphere and the relative 
positions of its component areas, remains comparatively con- 
stant throughout the history of the trochophore ; as does the 
prototroch itself. This offers avery satisfactory means of 
orientation with reference to the metamorphosis which takes 
place in the posterior hemisphere. 
Behind the prototroch occur the most significant changes, 
affecting the position of cells and areas, and also the form of 
the posterior hemisphere; these changes being coincident with 
the formation and development of the ¢rvunk. 
In the sixty-four-cell stage the ectoderm of the trunk, dorsal, 
lateral and ventral, is represented by the three cells of the Y 
group. These cells are restricted to the dorsal part of the 
posterior hemisphere, where they form a thick plate (somatzc 
plate). A large part of the surface area lying immediately in 
front of the plate is occupied by the cells of the mesoderm 
and entoderm. 
There is a small segmentation cavity. 
A line, acé, (Fig. II) passed through the trochophore from 
the apical pole, perpendicular to the prototroch, will fall in the 
area occupied by the mesoderm cell. I will call this line the 
vertical axis. # represents the morphological posterior end of 
the body. Thus, the antero-posterior axis, acf, does not coin- 
cide with the vertical axis, but is slightly bent. 
