THE DEVELOPMENT OF ASTERINA GIBBOSA. 345 
and in fig. 24 we see that the ceelom has grown back in the 
form of two tongues, Jpe., rpc., lying one at each side of the 
gut. Fig. 25 shows us a more ventral section passing through 
the blastopore of the same individual, and we see that in it 
these coelomic lobes are absent ; they are therefore still con- 
fined to the dorsal side of the embryo. 
It has been mentioned above that the larva, immediately on 
escaping from’ the egg-membrane, has the form of Stage B, and 
it will be observed that its anterior end has the appearance of 
being obliquely truncated, and that the anterior surface so 
constituted is surrounded by a thickened rim, which is covered 
with specially long cilia, and to which I give the name of larval 
organ. The changes of form involved in acquiring this shape 
are considerable, and are undergone whilst the larva is still 
enclosed in the egg-membrane, though superficially the ovoid 
shape is maintained, the larval organ and the neighbouring 
ectoderm being to a large extent developed as invagina- 
tions into the interior of the larva, exactly as the 
Tenia head is developed on the wall of the cyst. 
The histology of the embryo is illustrated in Plate 26, figs. 
124 and 125. The first is a portion of section of a larva of 
Stage A, the same specimen as that from which figs. 20 and 21 
are taken. Both ectoderm and endoderm are seen to consist of 
long narrow cylindrical ceils, and there is no mesenchyme. 
Recent researches have gone to show that this is exceptional. 
Field (5) has proved for Asterias, and it has been long known 
in the case of Echinids, that mesenchyme is formed by the wall 
of the blastula before any invagination has taken place. Fig. 
125 is taken from a slightly older gastrula. It shows the forma- 
tion of the mesenchymatous cells by the division of the endoderm 
cells. I found no indication that mesenchyme continued to be 
formed when Stage B is reached. The anterior wall of the 
celom is the spot where its formation lasts longest, as in 
Antedon (18). The coelomic epithelium consists of small cubical 
cells (see Pl. 23, fig. 95). 
We must now return to Stage B, up to which we have traced 
the development. A stomodzeum is now developed just behind 
