. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. [108] OVUM. pally its outer part. The vitelline membrane does not exist at first, but seems to be formed at a later period by the consolidation of an external layer of the primitive yolk substance. The time of the disappearance of the ger- minal vesicle has not been determined in many of these Mollusca. Previous to seg- mentation a phenomenon occurs, which has now been observed in a large number of ani- mals, but which first attracted special atten- tion in the gasteropodotis Mollusca; viz., the separation of one or more
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. [108] OVUM. pally its outer part. The vitelline membrane does not exist at first, but seems to be formed at a later period by the consolidation of an external layer of the primitive yolk substance. The time of the disappearance of the ger- minal vesicle has not been determined in many of these Mollusca. Previous to seg- mentation a phenomenon occurs, which has now been observed in a large number of ani- mals, but which first attracted special atten- tion in the gasteropodotis Mollusca; viz., the separation of one or more clear hyaline liquid globules of considerable size from the surface of the yolk substance, into the space be- tween it and the vitelline membrane. This was first observed by Dumortier*, and de- scribed by Pouchetf, by Van Beneden in the Aplysia {, by Nordmann in Tergipes Ecl- wardsiijji, by C. Vogt in Action ||, and by various others. A precisely similar phenome- non has also been observed in some of the Vertebrata, as in Mammalia by Wharton Jones, Barry, and BischofF, and in Batrachia by Newport. But though this separation of one or more hyaline globules from the yolk- substance at the time of segmentation appears to be a very general accompaniment of that process, it must be confessed that its import, either in connection with fecundation or de- velopment, has not yet been ascertained. Acephala. — In Acephalous Mollusca the ova are generally of small size, the yolk-sub- stance principally finely granular, the germinal vesicle clear, with a distinct macula, which last not unfrequently presents the form of a double or elongated biscuit-shaped particle. The vitelline membrane is distinct and possesses considerable strength ; and there is generally a considerable space occupied by clear fluid between it and the surface of the yolk. The most interesting feature of the ova of these Mollusca is the funnel-shaped aperture which most of them possess, leading through the vitelline
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