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 witli 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 

 viteUus, in aU 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 viteUus 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 atrimn. As there 

 are no cUia 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 Fyrosoma {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 strise 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. (Fig. 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 on the 

 occurrence of this contact, I cannot pretend to say v/ith certainty, but I doubt it ; for, as 

 will be seen, though very remarkable changes take place shortly after impregnation, 

 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 testis 

 of zooids whose ovisacs are in the stage under consideration (^Fi/rosoma resembling Salpa 



