236 PROFESSOR HUXLEY ON THE ANATOMY 



elseoblast has a length of about yV^h of an inch. The rudiment of tlie atrial aperture 

 (round, and about s^o^h 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 \^ith, 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 ij-a-rd of an inch, while the combined cyathozooid and ovisac are 

 -g-'oth of an inch long, the neural boundaries of the ascidiozooids project a little way beyond 

 the open end of the cyathozooid. The upper edges of their atrial apertures, now yi o^h 

 of an inch in diameter, are still fully ^xoth of an inch below the margin of the cyatho- 

 zooid ; and although the formation of the true cloacal chamber has commenced by the 

 separation of the test from its cyathozooidal mould, yet its depth is so slight (not more 

 than 3^0 th 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 iVth of an inch in diameter lias the combined cyathozooid and ovisac not 

 more than -^gtli 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 ^th of an inch in long 

 diameter) are vertically oval, g^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 -g-fth of an inch deep. 



The stolons of this foetus are -3 Jeth of an inch long, and are directed towards the 

 aperture of the cloaca. 



In one of the most advanced foetuses I have met with (PI. XXXI. fig. 15), about Ysth 

 of an inch in diameter, the greatest length of the ascidiozooids (or the diameter parallel 

 to the foetal axis) was -g^-nd of an inch, while their antero-posterior diameter was i^.jnd of 

 an inch. The long diameter of the combined ovisac and cyathozooid (the latter being 

 now completely hidden between the hcemal moieties of the ascidiozooids) was only -Tij-th of 

 an inch ; or, in other words, they had not a third of their former dimensions. Eacli 

 ascidiozooid of this foetus has a roughly semicircular profile, the sti-aight side being turned 

 towards the axis of the foetus. The curved contour is more convex on the haemal, more 

 flattened upon the neural face. From side to side each ascidiozooid is much compressed, 

 so as not to measure more than -5-Hth of an inch in this direction. 



The oral aperture is not yet pervious ; but a circular groove of the outer surface of 

 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 hsemal tentacle, -axo^h of an inch long, exhibits its characteristic enlarged base 

 and finger-like process. The peripharyngeal ridge exhibits its distinctive structure. 

 Hathcr in front of its upper loop, a small process (the upper end of the diapharyngeal band) 



