CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 24 
16-cell stage, and was foreshadowed still earlier by the internal constitu- 
tion of the unsegmented ovum. Wes endoderm fundament is in the sixth 
generation! (Plato XI. Fig. 71, d%, q**, d%, a, af”, and the corre- 
sponding cells in the left ee of the hiba the chorda-mesenchyme 
ring is chiefly in the seventh generation, though a single pair of it& cells 
has recently passed into the eighth (07%, DE dB, qti, ASP, AN, 
Tel A a 
— tho deep-lying sister cell of 4%, not shown in the Figure, — « 
7.10 
Qu 
a, and the corresponding cells in quadrants B and C) ; the cells 
of the neuro-museular ring are all in the eighth generation, except a 
single s which lingers in the seventh (D™ De, Je, D AnS, 
AS, 4*7, and the corresponding cells in quadrants DB aud C); the 
> 
E va are all in the ninth generation, those nearest the animal 
pole having been the first to divide and pass into that generation. We 
notice in this stage, as in tho earlier stages, a region of delayed division 
in the equatorial band at the posterior end of the embryo. 
B. HISTOLOGICAL, 
Figures 63-70 (Plate X.) represent eight, cross sections from a series 
through an embryo (not figured) a little more advanced in development 
than the one shown in Plate X. Figs. 61 and 62. The approximate 
position in the embryo of tho sections figured is indicated by the hori- 
zontal lines on Figure 62. 
The differing stainability of cells at this stage, together with other 
histological peeuliarities, serves already to distinguish the fundaments 
of the various organs with considerable precision. 
The endoderm cells (4%, Fig. 64; d99, Figs. 65 and 66; als, des, 
Fig. 67 Fig. 63; together with the corresponding cells in the left 
halves of theso Figures) are, on account of their slow division, still very 
large. They are columnar in form, and cont re nuclei. Then 
eytoplasmie portion seareely stains at i he reeion of th 
nuclei, being almo ntirely taken up with close-p lk eran 
mall amount of protoplasm staining a bri due in ) 
extends out from either side of the nucleus in the long axis of the 
1 thi p! toplasmi eviden 1 1 1 phere « 
) he wa { nt u hem ( i nu 
hromati ranu 
The mesenchyme cells (D7, Fie. 6 ( 
with the corresponding eells in the 
t See the table on page 275 
