AND DEVELOPMENT OF PYROSOMA, 
221 
cellular in structure, but it is devoid of any structureless investment or membrana propria. 
The contained ovum consists of a solid-looking, well-defined germinal spot s^th of an inch 
in diameter, occupying the centre of a germinal vesicle -g^th of an inch in diameter, with 
thin but well-defined wall, and perfectly clear 
The yelk is represented by 
small zone of structureless, yellowish substance, which invests the germinal vesicle, and, 
on the one hand, passes into the wall of the ovisac, while, on the other, it is separated 
from that wall by a narrow clear space. 
Second Stage. Ovisacs less than -^th of an inch in diameter and unimpregnated. 
The ovisac represented in fig. 2. exemplifies this condition very well. It has a dia- 
Its form is spheroidal, and it is produced on the side towards 
meter of 
3 
-tk of an inch 
the atrial wall of the blood sinus in which it lies, into a short subcylindrical diverticulum 
which is directed forwards, and slightly towards the 
al side of 
ascidiozooid 
which it lies 
This diverticulum, or rudimentary duct 
4 
of an inch in len 
and its slightly narrowed anterior extremity passes into the atrial tunic. At its opposite 
end, where it becomes continuous with the ovisac, it measures aroth of an inch in dia- 
meter. At this extremity, the cavity of the duct is in free communication 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 cavitv ceases, the termination of the duct appearing to be a solid 
cellular 
mass. In this condition, therefore, there would appear to be 
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 5 J th of an inch in diameter, imbedded in, and 
connected together by, a structureless substance. The wall of the duct is similarly com- 
posed, but its haemal is much thicker than its neural wall. In the cavity of the duct 
othin 
5 
clear fluid 
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 ^J-g-th of an inch 
m diameter, within which lies a germinal vesicle Cbiirth) with perfectly clear contents, 
inclosing an opake, spheroidal, germinal spot (rAa th) . The yelk is in close contact with 
the inner side of the anterior wall of the 
the germinal vesicle is close 
face at the same point, and the germinal spot is applied to the inner surface of the ant 
rior 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 membrane in ova in this stag 
It may be doubtful whether the space between the w r all of the ovisac and the ovum is 
an artificial product. My observations upon the ovisacs of a fresh Pyn 
natural 
(Phil. Trans. 1851) lead me to adopt the latter hypothesis 
Third Sta 
S e - 
Ovis 
tcs under jitjth of an inch in diameter and in process of impregnation. 
(PI. XXX. figs. 3, 4, and 8*.) 
Of the two specimens in this stage which I have figured, the larger (fig. 3) is rather 
the less advanced. Its duct is longer than the diameter of the ovisac, and is not only 
