ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA — 375 
process on anything short of direct evidence. Such evidence, how- 
ever, is only to be obtained by the examination of young Pyrosomata 
but little larger than foetuses, none of which have come into my 
possession. 
$5. Summary and discussion of the results of the observations on 
the gamogenetic development af PYROSOMA 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 Celenterata, 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 sper- 
matozoa 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 increases in size. By this deli- 
quescence of the yelk the germinal vesicle is laid bare. 
4. The germinal vesicle adheres to a particular spot of the epithe- 
lial lining of the ovisac, close to the opening of its duct and, even- 
tually 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, spheroidal 
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 
