CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. Z00 
pore, but they are much larger than the nerve cells. They now lie lateral 
and posterior to the blastopore. (See Figs, 81-83 ; cf. Figs. 79 and 80.) 
The backward growth of the anterior lip of the blastopore has carried 
the crescent-shaped anterior chorda fundament (Fig. 71) from its origi- 
nal position to about the middle of tho embryo's dorsal surface (Fig. 81). 
It was seen in Figure 71 to consist of eight cells, which have now in- 
creased (Figure 78) to sixteen, and lie erowded together in two rows, one 
superposed above and overhanging tho other (ef. Figs. 79 and 8l. 
2 (Plate X.) and 71 (Plate NL) we suw that the two 
posterior chorda cells, viz. ec", d^", were separated from the ante- 
In Figures ( 
76 4^9 or their descend- 
rior chorda cells by the mesenchyme cells, 5 
ants. In Figure 82 we see that the descendants of BEAS A 
p» B23, 92% and the corresponding cells in quadrant 4) during the 
process of invagination have been pushed down to the level of the other 
mesenchyme cells, allowing the anterior chorda eells to come into contact 
c9?) above them. 
with the isolated posterior chorda cells (Fig. 81, e? 
The posterior chorda cells were scen to be in the seventh generation in 
Figure 71 (e™™, dé), At the stage represented in Figure 81, there is 
good reason to bolieve that they have divided and passed into the eighth 
generation, since every other cell of the dorsal hemisphere is known to 
have done so previous to that stage; thoy are therefore represented. by 
the cells c81, £922. (8-2, and d$2, the last named cell being hidden from 
view in Figure 81 by the overlying muscle cell. 
The endoderm cells still remain in the eighth generation, and num- 
ber twenty. "Their arrangement is made clear by an examination of 
Figures 81-83, in comparison with Figure 79, which represents a section 
near and parallel to the median plane of a slightly earlier stage.. Four- 
teen of the twenty endoderm cells abut on the median plane, and six are 
placed laterally toward the anterior end of the embryo. The median 
double row of cells consists of 0^! (Fig. A sag. BD u EUR 
lls in the right half of 
(16 TB (Fig, 83), and the corresponding ci 
the embryo. The nuclei do not appear in the centrally and posteriorly 
situated endoderm cells of Figure 83 beeause they lie in later (deeper) 
sections of the series, not figured (ef. Fig. 10) Only the narrow upper 
ends of the cells in question appear in Figure 85, which therefore gives 
no adequate idea of their size, but a correct idea of this may be had by 
an examination of Figure 79. The laterally situated endoderm cells 
are c'? (lig. 82), c^19, ¿7:15 (Fig. 83), and the corresponding cells in 
the right half of the embryo. 
From a series of cross sections through an embryo in about the same 
