. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. [118] OVUM. Annelida. According to this description, the first germs of the ova consist of minute ger- minal vesicles formed in the ovarian sub- stance; they soon acquire the single macula or nucleus. After undergoing some enlarge- ment, these germs fall into the abdominal cavity, and there acquire, by deposit round them, the clear primitive vitelline substance. In this substance opaque granules, which are at first colourless, are subsequently deposited; and as these extend outwards from the ger- minal vesicle, and accum
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. [118] OVUM. Annelida. According to this description, the first germs of the ova consist of minute ger- minal vesicles formed in the ovarian sub- stance; they soon acquire the single macula or nucleus. After undergoing some enlarge- ment, these germs fall into the abdominal cavity, and there acquire, by deposit round them, the clear primitive vitelline substance. In this substance opaque granules, which are at first colourless, are subsequently deposited; and as these extend outwards from the ger- minal vesicle, and accumulate in quantity so as to increase the bulk of the whole ovum, a delicate vitelline membrane is added exter- nally. The germinal vesicle attains a diameter of about -$\v", and its macula of ^-^3"; and when the several parts of the ovum which have been mentioned have appeared, and the yolk is now coloured, the whole ovum has a diameter of about 75-^". The superficial part of the yolk consists of minute coloured granules. Within this there are larger oil-like globules free of colour, and in the innermost part a somewhat viscous transparent fluid.* According to Leydig, the germinal vesicle in Piscicola becomes enveloped by a second vesicle or cell-wall before the formation of the yolk-substance ; but it is suggested by Leuckart that he may have been misled in this by the appearance often presented by the clear and somewhat highly refracting substance which in many animals precedes the formation of the opaque yolk. If" this is not so, the fact observed by Leydig would constitute a marked departure from the usual homological relations of the Rotifera. — Although most zoologists are now disposed, on the ground of the analogies in the most important parts of their general structure, to place the Rotifera among or close to the Articulate Worms, yet in some re- spects their mode of reproduction presents a marked correspondence with that of the lower Crusta
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