ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 377 
germinal vesicle, and even a certain progress in yelk-division, may 
take place without impregnation ;1 whence it may seem less strange 
than it appears at first sight, to suppose that the influence of the 
spermatozoa may be exerted, in some cases, not upon the yelk, nor 
upon the germinal vesicle as such, but upon the nascent blastoderm. 
3. The only animals which, so far as I know, present a condition 
of the yelk at all comparable to its liquefied and pellucid state in 
Pyrosoma, are Ascaris dentata, Cucullanus elegans, and Oxyurts 
ambigua. In these nematoid worms, the vitellus, according to 
Kolliker?, is represented only by a clear, transparent fluid containing 
a very few granules, and it takes no direct share whatever in the 
formation of the embryo. The vitellus seems to play an equally 
subordinate part in the great majority of the Ayr¢icu/ata, but in these 
animals it is commonly opaque and granular. 
4. If the ovisac of Pyrosoma be compared with the Graafian follicle 
of a mammal, the resemblance (notwithstanding their obvious differ- 
ences) of the two structures is marked ; and the manner in which the 
germinal vesicle traverses the epithelivm of the ovisac of Pyrosoma is 
singularly like the manner in which the mammalian ovum imbeds 
itself among the cells of the proligerous disk. A still closer parallel, 
perhaps, is presented by the bird’s egg, if we consider the mode in 
which its germinal vesicle (which at first occupies the centre of the 
future egg, and is contained in a primitive ovum surrounded by, 
at any rate, a rudimentary vitelline membrane) passes to the sur- 
face, and eventually lies immediately beneath the membrane which 
encloses the food-yelk 3. 
5,6, 7. The consideration of the phenomena enumerated under 
these heads opens up the whole vexed question of the fate of the 
germinal vesicle. 
Since the imaginations of Dr. Martin Barry have fallen into just 
discredit, most physiologists have more or less distinctly adopted the 
doctrine that the germinal vesicle and its contents lose their identity 
and disappear; and that the embryo-cells, whence the blastoderm 
arises, are new structures not directly derived from them. 
1 See Leuckart, art. ‘‘Zeugung,” Wagner’s Handworterbuch, iv. p. 958. What 
Leuckart says here about the Frog is not in accordance with the results of the careful experi- 
ments of Newport (Phil. Trans. 1851, p. 190), who arrivesat the conclusion that segmentation 
certainly does not take place in the unimpregnated ovum. Vogt’s case is not satisfactory, as. 
there is no counter evidence to show that impregnated ova would have developed under the 
circumstances. Bischoff’s observations on the Sow (Ann. des Sci. Nat. 1844), however, appear’ 
to be unexceptionable evidence. 
? Beitrage zur Entwickelungs-geschichte wirbelloser Thiere. Muller's Archiv, 1843. 
3 See Dr. A. Thomson’s admirable article ‘‘ Ovum” in Todd’s Cyclopedia. 
