240 PROFESSOR HUXLEY ON THE ANATOMY 



however, is only to be obtained by the examination of young Pi/rosomata but little larger 

 than foetuses, none of which have come into my possession. 



§ 5. Smnniary and discussion of the results of the observations on the gamogenetic 

 develojmient of Pyuosoma gigakteum. 



If the observations detailed in the preceding pages be correct, and no flaw be found 

 in that interpretation of them which has been offered, it follows that — 



1. The ovisac of Fyrosoma at tu'st contains an ovum, altogether similar to that of 

 other animals, and in particular resembling that of many Ccelenterata, IfoUuscoida, and 

 Ilollusca, in the absence of a vitelline membrane. 



2. Impregnation is effected by the passage of the spermatozoa up tlie duct of the 

 ovisac, and it would seem that these spermatozoa must immediately come into contact 

 with the yelk ; but when, and how, the essential act of fecundation (consisting in the 

 action of the spermatozoa upon the germ) takes place, does not appear. 



3. There is neither complete nor partial, yelk-division ; but the vitellus disappears, as 

 such, apparently becoming diffused through the contents of the ovisac, which rapidly in- 

 creases in size. By this deliquescence of the yelk the germinal vesicle is laid bare. 



4. The germinal vesicle adheres to a particiilar spot of the epithelial lining of the ovi- 

 sac, close to the opening of its duct and, eventually traversing that epithelial lining, 

 takes up a position between it and the membrana propria of the ovisac. 



5. In the meanwhile, a turbid deposit takes place in that moiety of the germinal vesicle 

 which lies nearest the aperture of the duct ; and the germinal spot, a remarkably obvious 

 structure in this and preceding stages, is partially imbedded in this deposit. 



6. The germinal vesicle grows and becomes more flattened ; but soon, although it is 

 quite translucent, the germinal spot can no longer be found in it. In the place of that 

 structure and resulting, as I suppose, from its division, a number of small, clear, sphe- 

 roidal corpuscles are visible upon the face of the deposit in the germinal vesicle. 



7. Next, the germinal vesicle, as such, is no longer visible; but, occupying the same 

 place, preserving the same colour, having very nearly the same size and, on the side 

 turned towards the duct, the same curved contour, there is a flat patch, consisting of a 

 single layer of excessively delicate corpuscles, each with its clear space and central par- 

 ticle — constituting the commencement of the blastoderm. 



8. The blastoderm enlarges, assumes a band-like form, and becomes divided by con- 

 strictions into five segments : of these, one becomes the cyathozooid — a temporary 

 structuxe, which is especially attached to one pole of the ovisac, and, among other pur- 

 poses, serves as a sort of precursor, or mould, of the cloaca ; the other four are converted 

 into ascidiozooids. 



9. The ascidiozooids enlarge and eventually give rise to the tetrazooidal foetus described 

 by Savigny. The cyathozooid and the ovisac which it surmounts diminish in size and, 

 probably, eventually disappear. The lining epithelium of the ovisac early acquires a 

 pecu-liar vesicular structure. 



10. All these changes, subsequent to the formation of the blastoderm, take place in the 



