230 PROFESSOR HUXLEY ON THE ANATOMY 
embryo proper, as the foetus, reserving the term embnjo 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 ^ 3 -rd of an inch long by -^th of an inch wide, 
and so opake 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. 
Foetuses 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 ovi- 
sac and has, at the same time, become indented at opposite points of its margins, so as to 
be marked out into five short segments. One of the two terminal segments becomes much 
enlarged, spreading over and investing one pole of the ovisac like a cup ; while the other 
four remain far smaller, and, the indentations between them deepening, they are 
eventually connected only by narrow isthmuses of blastoderm. These segments are the 
rudiments of as many zooids ; but the large cup-like one has a totally different fate from 
the rest, and for distinction's sake I shall term it the cyathozooid, while the others are, in 
their order of nearness to it, the 1st, 2nd, 3rd, and 4th ascidiozooids * respectively. The 
zooids are not merely connected with one another by the isthmuses of blastoderm above- 
mentioned, but the structureless test has greatly increased in thickness, and now invests 
them all, like a thick layer of transparent varnish. The membrana propria of the ovisac 
is no longer distinguishable outside this rudimentary test. 
The remains of the duct are often still traceable, towards the conclusion of this stage, 
at one end of an equatorial diameter of the foetus (supposing the cyathozooid to be situated 
at one of its poles) ; but later, it is no longer to be discovered. 
Eighth Stage. Foetuses from -j%th of an inch up to the largest which have been met with. 
In describing this final stage of development, it will be convenient to consider, first, 
the changes in general arrangement, size and form, of the different parts of the foetus ; 
and secondly, the special modifications which each of these parts undergoes. 
The cyathozooid, at first, occupies but a comparatively small segment of the surface oi 
the spheroidal foetus, and the slightly curved series of ascidiozooids stretches out from it, 
over about half the circumference of the uncovered portion of the ovisac (PI. XXXI. ng. 
11). But, by degrees, the cyathozooid extends so far as to invest nearly half the surface 
of the ovisac, and, at the same time, the chain of ascidiozooids (considered as a whole) 
gradually assumes a new direction, and applies itself closely to the face of the cyathozooid 
whose circumference it half encircles (fig. 13). The blastoderm of the ascidiozooids, 
however, remains perfectly distinct from that of the cyathozooid, the two being unitec 
only by the layer of test, which, in the earlier stages, invested both, and w r hose con- 
tiguous edges now seem to run into one another. 
* I have, throughout the present memoir, used the term ' ascidiozooid,' as more euphonious than ' asci i > 
employed in my notice in the • Annals of Natural History' for 1860. 
