236 PROFESSOR HUXLEY ON THE ANATOMY 
elseoblast has a length of about g^th of an inch. The rudiment of the atrial aperture 
(round, and about aioth of an inch in diameter) lies altogether below the level of the 
equator of the combined cyathozooid and ovisac. There is an indistinct appearance as of 
a small cavity between it and the latter organs. The posterior end of the endostyle 
appears quite distinctly to be continued back into the central canal of the isthmus. The 
rudiment of the heart is obvious, in close connexion with, and apparently developed 
from the wall of the branchial sac ; and there are two slight papillary elevations in the 
place whence the stolons will be given off. 
In a foetus of about the same diameter as the preceding but whose ascidiozooids have 
a vertical diameter of ^ 3 -rd of an inch, while the combined cyathozooid and ovisac are 
jth of an inch long, the neural boundaries of the ascidiozooids proj ect a little way beyond 
the open end of the cyathozooid. The upper edges of their atrial apertures, now yi o tn 
of an inch in diameter, are still fully afoth of an inch below the margin of the cyatho- 
zooid ; and although the formation of the true cloacal chamber has commenced by the 
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separation of the test from its cyathozooidal mould, yet its depth is so slight (not more 
than 3yoth of an inch) that the end of the tongue-like inward prolongation of the test 
still lies between the lips of the mouth of the cyathozooid. 
A foetus of xi-th of an inch in diameter has the combined cyathozooid and ovisac not 
more than -g^-th of an inch long, and cup-shaped — its upper, open end being as broad as its 
middle. The atrial apertures of the ascidiozooids (which measure yoth of an inch in long 
diameter) are vertically oval, ^th of an inch long, and lie almost wholly above the 
level of the upper edge of the cyathozooid. They open at once into the cloacal cavity, 
which, as measured from its roof, formed by the now hardly-projecting tongue-shaped 
process, to the upper edge of the cyathozooid, is -^jth of an inch deep. 
The stolons of this foetus are 3^gth of an inch long, and are directed towards the 
aperture of the cloaca. 
In one of the most advanced foetuses I have met with (PL XXXI. fig. 15), about i^-th 
of an inch in diameter, the greatest length of the ascidiozooids (or the diameter parallel 
to the foetal axis) was ^nd of an inch, while their antero-posterior diameter was ^g-nd °* 
an inch. The long diameter of the combined ovisac and cyathozooid (the latter being 
now completely hidden between the haemal moieties of the ascidiozooids) was only Ts^h. ol 
an inch ; or, in other words, they had not a third of their former dimensions. Each 
ascidiozooid of this foetus has a roughly semicircular profile, the straight side being turned 
towards the axis of the foetus. The curved contour is more convex on the haemal, more 
flattened upon the neural face. Prom side to side each ascidiozooid is much compressed, 
so as not to measure more than -^Qth of an inch in this direction. 
The oral aperture is not yet pervious ; but a circular groove of the outer surface oi 
the test, ^th of an inch in diameter, indicates the area in whose centre it will appear, 
around which centre lie the oral sphincter and the tentacular fringe. The latter, at 
present, not only projects into the buccal cavity but is divided into its processes; and 
the haemal tentacle, sio^ 0I> an i ncn l° n g> exhibits its characteristic enlarged base 
and finger-like process. The peripharyngeal ridge exhibits its distinctive structure. 
Rather in front of its upper loop, a small process (the upper end of the diapharyngeal band) 
