ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 361 
Seventh Stage. Ovzsacs from 3;th to };th of an inch in diameter, in 
which the blastoderm rapidly increases, and becomes segmented into 
the rudiments of five zootds. 
Up to this stage the ovisac lies within the sinus system of the 
parent, which, as I have already pointed out, becomes accommodated 
to its increased dimensions, partly by the thrusting of the atrial tunic 
into the cavity of the atrium, but, to a much greater extent, by the 
formation of a chamber in the test, in consequence of the extension 
outwards of a diverticulum of the outer tunic. In the recent con- 
dition, the blood of the parent must circulate in the narrow space 
left between the walls of the ovisac and those of its containing 
chamber ; and it seems reasonable to suppose that the former im- 
bibes into its interior a supply of nutritive material, which will con- 
tribute towards the subsequent development of the embryo. 
But during and after this stage, the ovisac bearing the embryo is 
to be found loose in the mid-atrium, which, in its later stages, it fills. 
To arrive at this position it must necessarily break through the wall 
of the atrium or atrial tunic, and through the duct which still con- 
nects it with that tunic. The latter process is easily intelligible, 
considering the very small relative size and delicacy of the duct; 
but I confess I do not understand how the rupture of the atrial tunic 
can be effected without serious hemorrhage. However, the zooids 
in which the detached ovisacs have attained a large size appear to be 
in as good condition as any of the rest. 
Henceforward I shall speak of this complex body, composed of 
the ovisac and the embryo proper, as the /fwéus, reserving the term 
embryo for the blastoderm and the results of its modification. In 
such a foetus as that represented in fig. 10, the blastoderm is a broad, 
elongated, membranous patch jyrd of an inch long by ;Asth of an 
inch wide, and so opaque as at once to strike the eye when the foetus 
is viewed with even a very low power. It is composed of somewhat 
coarse, granular-looking corpuscles, and lies between the membrana 
propria and the modified epithelium; but the former is separated 
from it by a very thin layer of structureless substance which extends 
for some little distance beyond the limits of the blastoderm on each 
side. The further course of development shows that this layer is the 
rudiment of the test of the future ascidiarium. 
Feetuses of very slightly increased or even of less size exhibit a 
marked change in the embryo, which has elongated sufficiently to 
extend over half the circumference of the ovisac and has, at the same 
time, become indented at opposite points of its margins, so as to be 
