GASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 257 
blastopore (cf. Plate XI. Fig. 80). On its depressed dorsal surface are 
seen six cells of the anterior chorda fundament, which is being rapidly 
covered over from the sides and anterior end by the ectoderm. Extend- 
ing deep down on either side of the chorda appears a deeply stained cell 
(stippled in the drawing) with large nucleus. These two are the most 
posterior cella of the medullary plate, which now lies at the dorsal surface 
of the embryo anterior to the blastopore, having been formed chiefly by 
the anterior segment of tho neuro-muscular ring (cf. Plate XI. Fig. 80). 
The mesenchyme cells, A, B9 (Wig. 87), are seen to lie on each 
side of the gastrula cavity (cf. Plate XI. Fig. 82) ; lateral to them lie the 
relatively small and faintly stained mesenchyme cells, 492. 2034 he 
respective sister cells of those just mentioned, viz. 49%, 59-9, Are ama 
poa (cf. Plate XI. Fig. 82), lie in the next two posterior sections (not 
figured). 
The stage next to be discussed differs in external appearance from that 
shown in Figure 80 chiefly, first, in the further contraction of its blasto- 
pore to a small aperture in the dorsal surface somewhat posterior to its 
centre ; secondly, in a slight elongation of the embryo and narrowing of 
ita posterior end, foreshadowing the formation of the tail; aud thirdly, 
in a slight depression of tho medullary plate to form a neural or medul- 
lary groove (cf. Fig. 98). 
From a series of transverse sections through an embryo in this stage, 
five are represented in Plate XII. Figs. 88-92. Figure 88 (Plate XII.) 
shows a section posterior to the blastopore (of. Fig. 98). It passes 
through the region of the small posterior mesenchyme cells, AAA 
Lateral or dorsal to them are seen four pairs of muscle cells containing 
large nuclei. ‘The finely granular cytoplasm of these musele cells takes 
a deep grayish blue stain in himatoxylin. Bounding the whole section 
is the uninterrupted ectoderm. 
ho next anterior section, which has nearly twice the area of this, 
is likowise completely surrounded by ectoderm. 
‘The second section anterior to the one shown in Figure 88 is repre- 
sented in Figure 89 (Plate XIL.). Two endoderm cells, the most pos- 
terior ones, appear in it. The small size of their nuclei indicates that 
they belong to a later generation than the endoderm cells seen in Fig- 
uro 84, Unquestionably they are in tho eighth generation. To right 
and left of them appear two muscle cells, probably descendants of OT, 
D'^, Plate XI. Fig. 73 (cf. Plate XI. Bias. 0.2 Wares ze, and 
D*?), Lateral to the muscle cells mentioned are seen in Figure 89 
