CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 259 
next two anterior sections. Of the eight cells mentioned, A", 49% 
39. and 39? (cf. Vig. 82) are all in mitosis, but the four moro laterally 
situated and smaller ones are still quiescent. 
Considering as a whole the mesenchymo of this embryo, we see that it 
consists of two lateral bands which have elongated with the elongation 
of the embryo. They now extend through ten dilferent sections from 
near tho anterior end of the embryo to a region posterior to the blasto- 
pore (Fig. 89). The muscle cells, on the other hand, are gathered into 
a pretty compact mass at the sides of and posterior to the blastopore 
(Figs. 88-90). 
In the subsequent stages of development the portion of the embryo 
lateral and posterior to the blastopore will be rapidly drawn out to 
form the tail of the larva, while the portion «anterior to the blastopore 
will form the trunk. This will not come about, however, without a con- 
sidorable shiftine of cells from one portion into the other, for the chorda 
colls, which now lie anterior or lateral to the blastopore, must in large 
part pass imto the tail, while the mesenchyme cells, which are more ven- 
trally located, and some of which now extend behind the blastopore, 
will all pass forward into the trunk region. 
An examination of Figure 98 (Plate XII.) may help to give a clearer 
idea of tho stage just described. This figure shows a seetion made 
nearly parallel to the sagittal plane, but a little to one side of it, through 
an embryo slightly older than the one last under discussion (Figs. 88- 
92). The anterior chorda fundament, it is scen, has been carried back 
beyond tho middle of the embryo's dorsal surface. "The muscle cells 
have been forced backward and downward into a nearly vertical po- 
sition behind the blastopore, and are nearly covered. over with ectoderm 
(cf. Fig. 93). 
Numorous cell divisions have recently occurred in the ectoderm, and 
the number of endoderm cells has also plainly increased. A very marked 
elongation of the embryo has attended these divisions. Several cells in 
the medullary plate aro also dividing. On account of the slight obliquity 
of the plane o seetioning, the small posterior cells O75, Di (ms’chy.), 
do not actually appear in this section as represented, but have been pro- 
jected there from the adjacent section. In that section tho endoderm 
extends back in a double row of cells into contact with O75, DTS, as at 
tho stages shown in Plate XI. Figs. 78 and 79. 
[n Figures 93-97 (Plato XIL) are represented five cross sections 
through an embryo in about the same stage as is shown in Figure 98. 
The approximate position of the sections in tho embryo is indicated. on 
