376 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
of excessively delicate corpuscles, each with its clear space and 
central particle—constituting the commencement of the blastoderm. 
8. The blastoderm enlarges, assumes a band-like form, and be- 
comes divided by constrictions into five segments: of these, one 
becomes the cyathozooid—a temporary structure, which is especially 
attached to one pole of the ovisac, and, among other purposes, serves 
as a sort of precursor, or mould, of the cloaca; the other four are 
converted into ascidiozooids. 
g. The ascidiozooids enlarge and eventually give rise to the tetra- 
zooidal fcetus described by Savigny. The cyathozooid and the ovisac 
which it surmounts diminish in size and, probably, eventually disap- 
pear. The lining epithelium of the ovisac early acquires a peculiar 
vesicular structure, 
10. All these changes, subsequent to the formation of the blasto- 
derm, take place in the mid-atrium of the parent, which the fcetus, at 
length, completely fills. There appears to be no placental connexion 
between the fcetus and the parent; but the nutritive matter contained 
in the large ovisac may well be supposed to pass into the sinuses of 
the cyathozooid and thence into those of the ascidiozooid, and thus to 
subserve the nutrition of the whole fcetus. 
In successively commenting upon the preceding paragraphs, I 
shall consider how far the embryogeny of Pyrosoma can be parallelled 
by that of other animals, and how far it offers exceptional peculiarities. 
1. I do not think that any one, acquainted with the structure of 
the ovarian ova of other Ascidians and of the A/o/usca generally, will 
entertain the slightest doubt that the parts called germinal spot, ger- 
mnal vesicle and vitellus, respectively, in the preceding pages, really 
have the nature I have assigned to them. The ovisac corresponds. 
with a single acinus of the ovary of other Mollusca and Molluscoida, 
and is altogether similar to the solitary ovisac of Sa/pa. 
2. The process of impregnation presents nothing anomalous ; but, 
as regards the act of fecundation, it is remarkable that the sperma- 
tozoa should so long remain aggregated in a mass in the upper end 
of the duct, without, to all appearance, penetrating into the cavity of 
the ovisac or into the substance of the yelk. Still more singular is 
that appearance of scattered, rod-like bodies, not unlike the heads of 
spermatozoa, upon and about the very young blastoderm. If I could 
feel thoroughly assured that these bodies are really the spermatozoa, 
I should be inclined to follow out to some length a series of con- 
siderations suggested by the fact, as to the essential nature and place 
of occurrence of impregnation. For the present, however, I will 
merely remind the reader that the so-called ‘disappearance of the 
