CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 239 
of cells. This band is interrupted at only one point on each side of the 
embryo, where a single cell (A75, 875) of the ectodermal group reaches 
up into contact with the cells of the dorsal hemisphere. From the equa- 
torial band just described are derived chiefly nerve cells and mesoderm 
cells. 
(c) 64-cell Stage. 
The completion of the divisions foreshadowed by spindles in the ecto- 
dermal group of cells of the stage last discussed (Plate X. Figs. 57 
and 58) doubles the number of cells in that group, and brings the num- 
ber in the entire embryo up to sixty-four, distributed as follows. 
Ventral hemisphere (designated by the letters A, B, €, D) : — 
32 cells in the Sth generation — the ectodermal group. 
16 E Tth E = the equatorial band. 
48 
Dorsal hemisphere (designated by a, b, c, d) : — 
16 cells in the 6th generation. 
64 
Such a stage is shown in Plate X. Figs. 59 and 60, the former 
representing a ventral and the latter a dorsal view. "The egg has again 
assumed the flattened form which it had at the 32-cell stage. 
Examining first the ventral surface (Fig. 59), we see that the divisions 
foreshadowed in the 48-cell stage (Figs. 57 and 58) have in every in- 
stance occurred in a direction perpendicular to that of the spindle in 
the mother cell, though a slight displacement is in some cases appear- 
ing among the daughter cells, on account of the mitoses arising in the 
equatorial band. The cells of the ectodermal group, on account of their 
recent division, now number thirty-two, as many as are found in both 
the other groups put together. They are in the eighth generation, one 
generation in advance of the cells of the equatorial band, and two gen- 
erations in advanee of the cells of the dorsal hemisphere. "They are 
A491. 489 489 480, 483, 4814 and 791-789 together with the correspond- 
ing cells in quadrants Band C. 
The equatorial band is, as at the last stage, composed of sixteen cells 
all in the seventh generation, but six of them (three on each side of the 
median plano, Fig. 60, 47, 475, and D"*) now show signs of approaching 
division. Four of these mitotic cells form the anterior segment of the 
equatorial band, and are destined to produce a considerable portion of 
