372, ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
A cellular mass, ;}5th of an inch long, is attached to the external 
tunic, close to the end of the endostylic cone, if not directly 
connected with it; and this, I am inclined to think, is the rudi- 
ment of the generative blastema. I have not been able to detect 
any distinct structure (as of an ovisac or testis) in it, which is 
remarkable when one considers the early appearance of the ovisac 
in the buds. 
The branchial stigmata are altogether twelve in number. The 
anterior and posterior are rudimentary while most of the others 
extend across almost the whole depth of the branchial sac. The 
cilia are perfectly distinct upon their edges. The longitudinal 
branchial bars are nine in number. The intestine has nearly the 
same form as in the adult, and the tubular, hepatic system is well 
formed. The heart is visible in its place. The elzoblast is a mass 
of clear reticulated tissue, causing the hamal wall to bulge a little on 
each side of the middle line, and occupying the interval between the 
endostyle and generative 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 ~;nd of an inch and 
a breadth of 745th 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 propor- 
tional size of these oval apertures, whose long diameters are parallel 
with the axes of the fcetus, the intervening wall of the cloaca is very 
narrow. 
The cyathozooid and ovisac are 3th of an inch long by jth wide 
and more cylindrical than cup-shaped. The aperture, still distinctly 
visible, has a diameter of ;},th of an inch; and as the cloacal 
chamber is now =;th of an inch deep, the margin of the aperture 
is but just on a level with the convex, neural margin of the ceso- 
phagus of any of the ascidiozooids. 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, 
gcth of an inch long by ygyoth 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 
cof the cesophagus. In the middle of their course these bands lie very 
near the lips of the atrial aperture. 
