224 PROFESSOR HUXLEY ON THE ANATOMY 
in the much more rapid advance towards maturity of the female, than of the male, organ, in 
each zooid), the spermatozoa which effect impregnation must be derived from another zooid 
if not from another ascidiarium. The latter alternative is not so improbable as it looks at 
first sight, if we consider that a current constantly sets through the body of each zooid from 
the oral to the atrial aperture, and so out at the cloaca. Hence, the spermatozoa which 
are poured by the vas deferens of any given zooid, in which the testis has attained its full 
development, into its atrium, must be almost immediately carried into the cloaca ; and as 
a powerful current is setting into the cloaca from every other zooid, it does not seem 
possible that the spermatozoa should make their way into any one of these zooids against 
it. But on the other hand, as the Pyrosomata live in great troops, the spermatozoa cast 
out of the cloaca of any one Pyrosoma 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 doc- 
rine of the rarity of self-fertilization even among hermaphrodite animals. 
Fourth Stage. Ovisacs from To^th to ^th of an inch in diameter, in which the yelk 
disappears and the germinal vesicle becomes fixed to the wall of the ovisac. Pigs. 
6— 6a. 
Figure 5 represents an ovisac -g^-th of an inch in diameter, and fig. 6 another of y^th of 
an inch. The first thing to be observed about these ovisacs is, that they have increased 
in dimensions disproportionately 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 Tso tn of an inch, and in the larger examples 
of this stage it appears shrunken and withered. The spermatozoa, however, are always 
visible in its upper dilated end (fig. 6 b), but sometimes 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 cor- 
puscles, which lie in the wall of the ovisac in this as in earlier stages, appearing to be thin- 
ner 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 con- 
sequence 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 acquainted with them, I mis- 
trusted 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 microscope, I could turn out 
the ovum entire, the vitellus beins: so firm and consistent as to retain its form (fig. 8*) ; 
