PRELIMINARY ACCOUNT OF THE CELL LINEAGE 
OF AMPHITRITE AND OTHER ANNELIDS. 
A. D. MEAD. 
Cleavage. — The egg of Lepidonotus is small and quite free 
from yolk, develops into a very active trochophore, and, as 
one might expect, the cleavage is extremely regular. The 
first furrow cuts in at the animal and vegetative poles at the 
same time. At the four-cell stage all the cells are of the 
same size, and in no way distinguishable one from the other. 
In the later stages (up to seventy cells or more) the quadrants 
which these cells represent also appear to be exactly similar, 
so that whatever phenomena take place in one quadrant take 
place, at the same time, in each of the others. The number 
of cells increases, in geometrical progression up to sixty-four, 
by the practically synchronous division of all the cells at each 
successive stage. The result is the attainment of two, four, 
eight, sixteen, thirty-two, and sixty-four-cell stages. Up to the 
sixty-four-cell stage every cleavage furrow cuts the meridian 
obliquely (spiral cleavage of Lang, Wilson, Kofoid, e¢c.), while 
the direction of the cleavage reverses with each successive 
division. 
With the attainment of the sixty-four-cell stage this regu- 
larity suddenly ceases. Some of the cells now acquire cilia 
and stop dividing forever (prototrochal cells); others do not 
divide for a long time; in others, which form the cross as in 
Nereis, the next cleavage plane coincides with the meridian ; 
still others, including the rosette cells, continue to divide, the 
direction alternating as before. 
An invaginating plate of eight cells is formed at the vegeta- 
tive pole. The cells are the four ‘‘macromeres,” and the four 
“‘micromeres of fourth generation.” 
At the thirty-two-cell stage the polar bodies usually pass 
into the apical cells, where they can be followed for a long 
time. They eventually disappear within the cells. 
