2i0 PROFESSOR HUXLEY ON THE ANATOMY 
however, is only to be obtained by the examination of young Pyrosomata but Utile larg 
than foetuses, none of which have come into my possession. 
§ 5. Summary and discussion of the results of the observations on the gamogenetic 
development o/Tyrosoma giganteum. 
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 Pyrosoma at first contains an ovum, altogether similar to that of 
other animals, and in particular resembling that of many Ccelenterata, Molluscoida, and 
Mollusca, in the absence of a vitelline membrane. 
2. Impregnation is effected by the passage of the spermatozoa up the 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 particular 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 
single layer of excessively delicate corpuscles, each with its clear space and central par 
tide — constituting the commencement of the blastoderm 
of 
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 
structure, 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 conver e 
into ascidiozooids. , 
9. The ascidiozooids enlarge and eventually give rise to the tetrazooidal foetus descri e 
by Savigny. The cyathozooid and the ovisac which it surmounts diminish in size an , 
probably, eventually disappear. The lining epithelium of the ovisac early acquires a 
peculiar vesicular structure. s , , 
10. All these changes, subsequent to the formation of the blastoderm, take place in 
