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 S(j 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 defex-ens 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 Tyrosomata 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 aziother, 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, Pyrosoyna affords a curious illustration of Mr. Darwin's doc- 

 rine of the rarity of self-fertilization even among hermaphrodite animals. 



Fourth Stage. Ovisacs from xo^th to 4^th of an inch in diameter, in which the yelk 



disappears and the germinal vesicle becomes fixed to the icall of the ovisac. Eigs. 



5— 8«. 



Figure 5 represents an ovisac -gVli 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 tI oth 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 Avell defined, but 

 that, by careful manipulation with needles, under the simple microscope, I could turn out 

 the ovum entire, the vitellus being so firm and consistent as to retain its form (fig. 8*) ; 



