354 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
any one Pyresoma may very readily be taken in by the oral aperture 
of another, and passing with the current through the branchial 
stigmata into the atrium, may easily reach the aperture of the 
oviduct. 
If this reasoning is valid, Pyrosoma affords a curious illustration 
of Mr. Darwin's doctrine of the rarity of self-fertilization even among 
hermaphrodite animals. 
Fourth Stage. Ovdsacs from yigth to Qyth of an inch in diameter, in 
which the yelk disappears and the germinal vesicle becomes fixed to 
the wall of the ovisac. Figs. 5—8a. 
Figure 5 represents an ovisac jth of an inch in diameter, and 
fig. 6 another of th of an inch. The first thing to be observed about 
these ovisacs is, that they have increased in dimensions dispropor- 
tionately to their ducts; for while, in the preceding stage, the duct 
is longer than the transverse diameter of the ovisac, in the present 
stage, it, at first, hardly equals, and subsequently remains much 
shorter than, that diameter. The duct, in fact, does not attain a 
greater length than ;4,5th of an inch, and in the larger examples of 
this stage it appears shrunken and withered. The spermatozoa, how- 
ever, are always visible in its upper dilated end (fig. 6 4), but some- 
times they no longer form a distinct bundle, but appear scattered, and 
then their rod-like heads are very distinct. 
In the wall of the ovisac and of the duct, a differentiation has 
taken place into an outer structureless membrana propria, and an 
inner epithelial layer. The latter is pale, the corpuscles, which lie in 
the wall of the ovisac in this as in earlier stages, appearing to be 
thinner and separated by wider clear interspaces. That change which 
arrests the attention of the observer most forcibly, however, is the 
entire absence in the present, as in all subsequent stages, of that 
vitelline mass which is so conspicuous in less advanced ovisacs. As 
a consequence of this disappearance of the yelk, the germinal vesicle 
lies apparently free and bare, in contact with one wall of the ovisac. 
There is not the slightest difficulty in observing these facts, nor the 
least ambiguity about the microscopical appearances ; but the circum- 
stances appeared so unprecedented, that, when I first became ac- 
quainted with them, I mistrusted the obvious interpretation of those 
appearances. However, I found, not only that the contour of the 
yelk contained in the smaller ovisacs was perfectly well defined, but 
that, by careful manipulation with needles, under the simple micro- 
scope, [ could turn out the ovum entire, the vitellus being so firm 
