CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 253 
position in the dorsal wall of the archenteron ; we may now observo that 
certain cells of the latero-postorior segments of the neuro-muscular ring 
are also involved in this ingrowth or invagination. At the stage shown 
in Figures 74-76 there was a double row of neuro-museular cells on 
each side of the blastopore, now (see Fig. 80) the inner row is nearly 
covered from sight by the row of cells outside it. 
Already at the stage shown in Figure 72 the most posterior neuro- 
muscular cells (07, 079) had moved from their originally lateral posi- 
tion toward the median plane (cf. Fig. 71). There they were destined 
presently to meet each other, covering over the small mesenchyme cells 
Ds, OTS (of. Figs. 71 and 72 with Fig. 80) ; now (Fig. 80) they or their 
descendants lie at the posterior angle of the blastopore, aud are in turn 
being covered over by the more laterally and anteriorly situated neuro- 
museular cells. 
The nerve cells anterior to the blastopore have increased considerably 
in number, perhaps through additions from the ectodermal group (cf. 
Figs. 72 and 80, also Figs. 78 and 79). 
Three sections from a horizontal series through an embryo of about 
the stage shown in Figure 80 are represented in Plate XI. Figs. 81-83. 
The series consists of thirteen sections 6.67 p thick, of which Figure 
81 represents tho third, Figure 82 the fifth, and Figure 83 the seventh. 
The sections are a little oblique, and consequently strike the right and 
left halves of the embryo at slightly different levels. The left side of 
Figure 82 shows best the history of the mesenchyme cells since the last 
stage examined in detail (Figs. 73-77). Lateral to the small gastral 
cavity we find the sister cells 5929, 592* descendants of the common 
mother cell B! (Fig. 77). Evidence of the derivation of these two cells 
has been cited in the observation of a spindle longitudinally directed in 
the cell 2*2 in two different embryos less advanced than this. 
Lateral to 499 and B%% are the sister cells B74, 5°, descendants 
of Bt (Fig. 71; cf AM, Fig. 77). They stain more faintly than 
1929 and B%% a distinction which, it will be remembered, existed 
between the respective mother cells 5*!! and BS (Fig. 77). Though a 
spindle has in no embryo been directly observed in BeH, 
sistership of BP and B*2 (Fig. 82) exists in the still persistent inter- 
evidence of the 
zonal filaments which stretch between their nuclei. This evidence is 
supported by the similarity of the cells in size and stainability. Poste- 
rior to the quartette of cells just discussed, tho common descendants of 
B'5 (Plate X. Figs. 62, 67), are the two daughter cells derived from 
c? (Fig, 71), which was seen to be mitotic at an earlier stage (Fig. 70). 
