238 PROFESSOR HUXLEY ON THE ANATOMY 



and the tubular, hepatic system is well formed. The heart is visible in its place. The 

 elseoblast is a mass of clear reticiilated tissue, causing the haemal wall to bulge a little on 

 each side of the middle line, and occupying the interval between the endostyle and gene- 

 rative blastema, on the one hand, and the heart and intestine on the other. 



The atrial aperture is enormous in proportion, occupying the greater part of the inner 

 face of the ascidiozooid above the level of the cyathozooid and attaining a length of fully 

 :^2nd of an inch and a breadth of xr^th of an inch. In other words, the atrial aperture is 

 six times as large as it is in the adult ascidiozooid, though the latter is at least eight or 

 ten times as large as one of the zooids of the foetus under description. In consequence 

 of the great proportional size of these oval apertures, whose long diameters are parallel 

 with the axes of the foetus, the intervening wall of the cloaca is very narrow. 



The cyathozooid and ovisac are -g^jth of an inch long by y^Q-th wide and more cylindrical 

 than cup-shaped. The aperture, still distinctly visible, has a diameter of -g^oth of an inch ; 

 and as the cloacal chamber is now j^-th of an inch deep, the margin of the aperture is 

 but just on a level with the convex, neural margin of the oesophagus of any of the asci- 

 diozooidsc Where the former tongue-like process existed, the roof of the cloaca now 

 hardly projects inwards at all. 



The atrial muscles are visible as very delicate, straight bands, ^^^-th of an inch long by 

 3^00^^^ wide, which take an oblique course on each side, from a point a little below the 

 end of the endostyle, neurad and a little forwards, to a point opposite the commencement 

 of the oesophagus. In the middle of their course these bands lie very near the lips of 

 the atrial aperture. 



The stolons are g-jotli of an inch long ; they pass almost horizontally inwards, towards 

 the rudimentary lip of the cloaca, and are curved towards its cavity, at their blind extre- 

 mities. The corpuscles of which their Avails are composed are more elongated than before 

 and, sending processes into the adjacent substance of the test, cause the .ca^cal ends of 

 the stolons to have a very pecuHar, brushlike appearance. 



3. The Test. — As I propose to reserve the description of the histological changes under- 

 gone by the embryo of Tyrosoma for another occasion, I will merely state, in this place, 

 that the test appears, at first, to be a structureless excretion. Subsequently, cellular 

 bodies, like connective-tissue corpuscles, are discernible in its most superficial layer, and 

 are disposed in such a manner as to form a very regular, hexagonal network, with large 

 meshes. The most advanced foetus has presented neither of the fibrous layers visible in 

 the adult test. 



Ninth Stage. The conversion of the tetrazooidal fa-tus into the adult ascidiarkmi. 



The most advanced foetus which has been described differs from tlie adult ascidiarium 

 not merely in size, in the paucity of its ascidiozooids, in the form and proportions of 

 the latter, in the absence of buds, or ever so slightly differentiated reproductive organs, 

 in them, and in their large atrial apertures (aU of which are peculiarities which we 

 may easily conceive to be altered by age and growth), but in still more important charac- 

 ters, seeing that in the adult ascidiarium I have met with no trace of the cyathozooid or 

 the isthmuses, nor have I been able to discover any ascidiozooid with two stolons. 



