Quarterly journal of microscopical science . e mantle. Outside this gelatinous layer is a hard mem-branous capsule, which in turn is studded with peculiarstructures, varying exceedingly in the various species. These,however, are of no importance and speedily disappear. The vitellus varies a good deal. In Phall. mammillatait is highly transj)arent and refractive; in Asc. intestinalis,Cynthia, &c., it is opaque, brown or dark yellow. No nucleus 60 can be detected in the mature ova, thougli it is readily seenin tlie immature. The Ascidians are true hermaphrodites, and artificial im-pregnation can
Quarterly journal of microscopical science . e mantle. Outside this gelatinous layer is a hard mem-branous capsule, which in turn is studded with peculiarstructures, varying exceedingly in the various species. These,however, are of no importance and speedily disappear. The vitellus varies a good deal. In Phall. mammillatait is highly transj)arent and refractive; in Asc. intestinalis,Cynthia, &c., it is opaque, brown or dark yellow. No nucleus 60 can be detected in the mature ova, thougli it is readily seenin tlie immature. The Ascidians are true hermaphrodites, and artificial im-pregnation can readily be conducted with ova and sperma-tozoa taken from the same animal. The spermatozoa attachthemselves to the ova not by their heads but by their entrance into the ovum was not observed. Cleavage commences within an hour of impregnation, andadvances very rapidly. It is confined entirely to the vitellus,the gelatinous layer Avith its vesicles taking no share in theprocess. Two meridian furrows divide the ovum first into. two then into four masses, which are further divided intoeight by an equatorial furrow. Even in the two first massesa nucleus may be detected, and the cleavage masses arrangethemselves around a central cleavage cavity (cavity of Baer). As soon as the number of cleavage masses amounts toabout thirty-two the ovum begins to be flattened below, andthen is doubled in on itself. Through this involution theovum becomes gradually transformed into a double sac, thecells from the upper set of cleavage masses, numerous andsmall, forming the outer sac, while those from the lower set,fewer and larger, forms the inner or lining sac, the originalcleavage cavity being reduced to a narrow space betAveenthe two, as is shoAvn in fig. 1 c. The new cavity thusformed corresponds to the alimentary cavity of the futureanimal {h); at first it is of the form of a shallow cup, but itgradually becomes deeper and its orifice narrower, until atlast if the external opening d
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Keywords: ., bookcentury1800, bookdecade1870, bookpublisherlondo, bookyear1870
