Prof. T. H. Huxley on the Development o/ Pyrosoma. 31 



still perfectly clear and transparent. The spot retains its pre- 

 vious form and dimensions. 



In these ovisacs I have been always able to find spermatozoa 

 either entering the mouth of the duct or, more usually, aggregated 

 together into a sort of conical bundle or plug, at the wide end 

 of the duct, where it opens into the ovisac. Not unfrequently 

 the broad end of this plug was in direct contact with the yelk 

 itself. I have assured myself of these facts by observing the 

 contents of the ovisac when opened with a fine needle, or, more 

 accidentally, by making thin sections with a razor ; and I have 

 satisfied myself that the bundle of filaments in question (itself 

 very obvious) consists of spermatozoa, by comparing them with 

 the contents of the deferent duct of the testis of Pyrosoma*. 



The spermatozoa must always come from some other zooid 

 (if not from another individual), inasmuch as the testis of 

 zooids containing ovisacs in this state is in a very rudimentary 

 condition. At no period, either before or after impregnation, 

 can any vitelline membrane be discovered \ but what surprised 

 me most was, that in ovisacs of larger size than r^th of an inch 

 in diameter the yelk itself was absent, at least in its previous 

 form. Not a trace of it is to be seen ; and I can only imagine 

 that it is completely liquefied and dissolved in the clear fluid 

 which forms the sole contents of the ovisac besides the germinal 

 vesicle. The latter is naked, and attached to the epithelial lining 

 of the wall of the ovisac (which is now differentiated into epi- 

 thelium and tunica propria) close to the opening of the duct 

 into it, and, so far as I have observed, always a little behind and 

 to one side of that opening. This constancy in the position of 

 the germinal vesicle becomes very important as a means of 

 identifying the germinal vesicle in subsequent stages of develop- 

 ment. Neither in this nor in any subsequent stage does the 

 germinal vesicle attain a mean diameter of more than T^th of 

 an inch, while ova in an earlier stage, surrounded by their yelks, 

 often measure T -goth f an inch, or fully double the size. Be- 

 sides this, the germinal vesicle is at first perfectly transparent 

 and clear, and the germinal spot, which retains its primitive size 

 and appearance, is recognizable in it with the utmost ease. It 

 is quite impossible, therefore, to confound the germinal vesicle 

 with the ovum of earlier stages, or the germinal spot with the 

 germinal vesicle of previous stages. 



That no solid yelk, such as previously existed, now invests the 

 germinal vesicle, is rendered obvious by opening the ovisac and 

 everting that part of its wall to which the germinal vesicle is 



* In my memoir on Pyrosoma (I. c. p. 584) I have described appearances 

 which led me to believe I saw moving spermatozoa in the duct in living 

 specimens. 



