AND DEVELOPMENT OF PYROSOMA. 
241 
mid-atrium of the parent, which the foetus, at length, completely fills. There appears to 
be no placental connexion between the foetus and the parent; but the nutritive matter 
contained in the large ovisac may well be supposed to pass into the sinuses of the cyatho- 
zooid and thence into those of the ascidiozooid, and thus to subserve the nutrition of the 
whole foetus. 
In successively commenting upon the preceding paragraphs, I shall consider how far the 
embryogeny of JPyrosoma can be parallelled by that of other animals, and how far it oilers 
exceptional peculiarities. 
1. I do not think that any one, acquainted with the structure of the ovarian ova of 
other Ascidians and of the Mollusca generally, will entertain the slightest doubt that the 
parts called germinal spot, germinal vesicle and vitellus, respectively, in the preceding 
pages, really have the nature I have assigned to them. The ovisac corresponds with ;i 
single acinus of the ovary of other Mollusca and Molluscoida, and is altogether similar 
to the solitary ovisac of Salpa. 
2. The process of impregnation presents nothing anomalous; but, as regards the act of 
fecundation, it is remarkable that the spermatozoa should so long remain aggregated in a 
mass in the upper end of the duct, without, to all appearance, penetrating into the eavity 
of the ovisac or into the substance of the yelk. Still more singular is that appearance 
of scattered, rod-like bodies, not unlike the heads of spermatozoa, upon and about the very 
young blastoderm. If I could feel thoroughly assured that these bodies are really the 
spermatozoa, I should be inclined to follow out to some length a series of considerations 
suggested by the fact, as to the essential nature and place of occurrence of impregnation. 
For the present, however, I will merely remind the reader that the so-called * disappear- 
ance of the germinal vesicle,' and even a certain progress in yelk-division, may take place 
without impregnation*; 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 dentate, Ctwullanus 
elegans, and Oxtjuris ambigua. In these nematoid worms, the vitellus, according to 
Kolliker f, 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 Articnlata, but in these 
animals it is commonly opake and granular. 
" 4. If the ovisac of Pyrosoma be compared with the Graafian follicle of a mammal, the 
resemblance (notwithstanding their obvious differences) of the two structures is marked ; 
and the manner in which the germinal vesicle traverses the epithelium of the ovisac of 
Pyrosoma is singularly like the manner in which the mammalian ovum imbeds itself 
agner 
Newport 
— -vw^^uu^v TTlUi LUC- IVOUitO Ul L11C I^CWV.1*** VA l/va *i"v**v^ ^- x x - 
at 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 circum- 
stances. Bischoff's observations on the Sow (Ann. des Sci. Nat. 1844), however, appear to he unexceptionable evidence. 
t Beitrage zur Entwickelungs-geschichte wirbelloser Thiere. Muller's Archiv, 1843. 
VOL. XXIII. 2 K 
