244 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
The equatorial band as a whole has now moved to a position distinctly 
within the margin of the dorsal surface (Fig. 71), so that a row of cells 
from the ectodermal group of the ventral hemisphere has come into view 
outside it round almost the entire periphery of the embryo (cf. Fig. 62). 
This change has come about in consequence of additional divisions in the 
ectodermal group of cells, which now not only has spread over the entire 
ventral surface of the embryo, but is encroaching upon its dorsal surface. 
Division has occurred nearly synchronously in all the cells of the ecto- 
dermal group, though somewhat sooner in those nearest the animal pole. 
(See Plate X. Figs. 63-70.) The strongest possible confirmation of my 
own observation regarding the simultaneousness of division in the cells 
of the ectodermal group in this period of development is afforded by 
Samassa’s (94) Figures 10 and 11, Taf. II. These represent respectively 
a dorsal and a lateral view of a stage intermediate between those shown in 
my Figures 62 and 71. In Samassa's Figures 10 and 11, all the cells of 
what I have called the equatorial band are figured as containing qui- 
escent nuclei, except the four seen to bo mitotic in my Figure 62 
(Samassa’s cells 3 and 6, Fig. 10). The other cells of the ventral 
hemisphere visible in Samassa's figures are without exception in process 
of division. It may accordingly be confidently assumed that at the stage 
shown in Figure 71 the cells of the ectodermal group of the ventral 
hemisphere have all passed into the ninth generation. If so, they number 
sixty-four ; this agrees well with the approximate count which one can 
make from dorsal and ventral views, though it is impossible to be sure 
about the exact number of ectoderm cells lying at this stage in an equa- 
torial position between the dorsal and ventral surfaces. Accordingly 1 
shall not attempt to give for this and subsequent stages the lineage of 
the individual cells of the ectodermal group. "This would be à work of 
great difficulty and of some uncertainty, for in this case the cells entirely 
lack those marked differences of size, stainability, and arrangement 
which make the lineage for the cells of the equatorial band and dorsal 
hemisphere a matter of perfect definiteness. 
If the estimate given of the number of cells in the ectodermal group 
is correct, the embryo shown in Plate XI. Fig. 71 represents a stage of 
one hundred and twelve cells distributed as follows. 
Ventral hemisphere : — 
64 cells in the 9th generation — the ectodermal group, 
20 i WO MF ) 
6 & Tth = j — the equatorial band. 
