AND DEVELOPMENT OF PYROSOMA. 
223 
manner, that the plane of the constricted junction between the duct and the ovisac is 
perpendicular to the stage of the microscope, so as to afford a true profile view, the broad 
end of the plug will appear to be in direct contact with the vitellus, close to the germinal 
vesicle. But I have never met with any such absolute contact in a true profile view. 
On the contrary, in such a view, the end of the plug appears to be jammed in the upper 
aperture of the duct, and there is a small interval between it and the surface of the 
vitellus. 
But it must be remembered that (as I have already pointed out) in the fresh state, the 
vitellus, in all probability, occupies the whole cavity of the ovisac, and itself stops the 
upper aperture of the duct ; and, if this be the case, it is exceedingly likely that the slight 
separation between the yelk and the plug of filaments is a post-mortem change. At any 
rate no filaments are ever discoverable in the cavity of the ovisac, and as I have been 
able to find no complete diaphragm across the upper aperture of the duct, there seems to 
be no reason for their absence, unless we suppose that the vitellus itself bars their 
entrance. But in this case the plug and the vitellus must come into direct contact. 
This point is of great importance, because there can be no doubt that the filaments in 
question are spermatozoa. The * plug,' and the contents of the vas deferens of the testis 
are precisely similar in appearance. The plug is not visible before the atrial end of the 
duct is open, thus providing free access for spermatozoa floating in the atrium. As there 
are no cilia on the inner surface of the duct, it seems impossible to account for the pre- 
sence of the dense mass of filaments within it, except on the supposition that they have 
an inherent propulsive power ; and the only free, filamentous bodies possessed of such a 
power we know of, in the animal economy, are spermatozoa. 
Furthermore, in my former memoir on Pyrosoma (I. c. p. 584), I have recorded the 
following observation : 
" In young specimens, when the ovum is small and the yelk pale, this gubernaculum 
[the duct] frequently appears to be solid ; but in fully grown specimens, when the ovum 
[ovisac] has its full size, and the yelk is dark and granulous, it presents the appearance 
of a wide tube, especially at its upper part. And here, there was frequently an appear- 
ance of dark striae and moving granules, prompting the belief that spermatozoa had 
travelled thus far. In one instance the sac of the ovum was empty, and the guber- 
naculum or duct widely distended ; the appearance of spermatozoa in the duct was here 
very strong. (Pig. 5.)" 
I entertain no doubt, then, that the specimens described exhibit the process of impreg- 
nation in Pyrosoma ; that the spermatozoa make their way up the duct and come into 
contact with the surface of the yelk. Whether that reciprocal action of the spermatozoon 
and the ovum, which constitutes the essence of fecundation, takes place immediately 
occurrence of this contact, I cannot pretend to say with certainty, but I doubt it ; 
will be seen, though very remarkable changes take place shortly after impregnatior 
they are not those which in other animals follow upon fecundation. 
It is not a little singular that, in consequence of the immature condition of the testi 
°f zooids whose ovisacs are in the stage under consideration (Pyrosoma resembling Sa/p 
for 
