AND DEVELOPMENT OF PYROSOMA. 241 



mid-atrium of the parent, which the fetus, at length, completely fiUs. 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 Pyrosoma can be parallelled by that of other animals, and how far it offers 

 exceptional peculiarities. 



1. I do not think that any one, acquainted with the structure of the ovarian ova of 

 other Ascidians and of the MoUusca 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 liave assigned to them. The ovisac corresponds with a 

 single acinus of the ovary of other Ilollusca and Molluscolda, and is altogether similar 

 to the solitary ovisac of Sal pa. 



2. The process of impregnation presents nothmg 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 aU appearance, penetrating into the cavity 

 of the ovisac or into the substance of the yelk. StiU more singular is that appearance 

 of scattered, rod-like bodies, not unlike the heads of spermatozoa, upon and about the very 

 young blastodeimi. 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 natiu-e 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 ajipears 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 liqvxefied and pellucid state in Pyrosoma, are Ascaris dentata, Cucullamis 

 elegans, and Oxyuris ambigua. In these nematoid worms, the vitellus, according to 

 Kolliker t, 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 Articulata, 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 difl^erences) 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 



* 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 experiments of Newport (Phil. Trans. 1851, p. 190), who arrives 

 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 be unexceptionable evidence. 



t Beitrage zur Entwickelungs-geschichte wirbelloser Thiere. Miiller's Archiv, 1843. 

 VOL. XXIII. 2 K 



