350 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
Second Stage. Ovtsacs less than zAgth of an inch in diameter and 
unimpregnated. 
The ovisac represented in fig. 2. exemplifies this condition very 
well. It has a diameter of 535th of an inch. Its form is spheroidal, 
and it is produced on the side towards the atrial wall of the blood 
sinus in which it lies, into a short subcylindrical diverticulum, which 
is directed forwards, and slightly towards the neural side of the 
ascidiozooid in which it lies. This diverticulum, or rudimentary 
duct, is ;4,th of an inch in length, and its slightly narrowed anterior 
extremity passes into the atrial tunic. At its opposite end, where it 
becomes continuous with the ovisac, it measures ,1,th of an inch in 
diameter. At this extremity, the cavity of the duct is in free com- 
munication with that of the ovisac, but at a little more than half way 
towards the opposite end, or in other words towards the atrium, the 
cavity ceases, the termination of the duct appearing to be a solid 
cellular mass. In this condition, therefore, there would appear to be 
no communication between the interior of the ovisac and the atrial 
cavity. 
The wall of the ovisac exhibits no distinct membrana propria, but 
is composed of a single layer of flattened corpuscles, about s,/g5th of 
an inch in diameter, imbedded in, and connected together by, a 
structureless substance. The wall of the duct is similarly composed, 
but its heemal is much thicker than its neural wall. In the cavity of 
the duct nothing save a clear fluid is discernible, and the same fluid 
seems to fill the interval observable on one side, between the wall of 
the ovisac and the ovum. 
The latter consists of a very finely granular, spheroidal vitelline 
mass ;}.th of an inch in diameter, within which lies a germinal 
vesicle (;!,th) with perfectly clear contents, inclosing an opaquer 
spheroidal, germinal spot (;g:5th). The yelk is in close contact 
with the inner side of the anterior wall of the ovisac—the 
germinal vesicle is close to its surface at the same point, and 
the germinal spot is applied to the inner surface of the anterior 
wall of the vesicle, so that it is as near as possible to the wall of 
the ovisac. 
I have not been able to discover a trace of a vitelline mem- 
brane in ova in this stage. It may be doubtful whether the 
space between the wall of the ovisac and the ovum is a natural 
or an artificial product. My observations rupon the ovisacs of 
a fresh Pyrosoma (Phil. Trans. 1851) lead me to adopt the latter 
hy pothesis. 
