The germinal vesicle. The germinal vesicle exhibits all the essential characters of a nucleus. It has a more or less spherical shape, and is enveloped by a distinct membrane which seems, however, in the living state to be very often of a viscous semi-fluid nature and only to be hardened into a membrane by the action of reagents (Fol). The contents of the germinal vesicle are for the most part fluid, but may be more or less granular. Their most characteristic components are, however, a protoplasmic network and the germinal spots[14]. The protoplasmic network stretches from the germinal spots to the investing membrane, but is especially concentrated round the former. (Fig. 5.) The germinal spot forms a nearly homogeneous body, with frequently one or more vacuoles. It often occupies an eccentric position within the germinal vesicle, and is usually rendered very conspicuous by its high refrangibility. In many instances it has been shewn to be capable of amoeboid movements (Hertwig, Eimer), and is moreover more solid and more strongly tinged by colouring reagents than the remaining constituents of the germinal vesicle. [14] In the germinal vesicles of very young ova the reticulum is often absent. In many instances there is only one germinal spot, or else one main spot and two or three accessory smaller spots. In other cases, _e.g._ Osseous Fishes, Echinaster fallax, Eucope polystyla, there are a large number of nearly equal germinal spots which appear to result from the division or endogenous proliferation of the original spot. Sometimes the germinal spots are placed immediately within the membrane of the germinal vesicle (Elasmobranchii and Sagitta). In many Lamellibranchiata, in the earthworm, and in many Chætopoda the components of the germinal spot become separated into two nearly spherical masses (fig. 12), which remain in contiguity along a small part of their circumference, and are firmly united together. The smaller of the two parts is more highly refractive than the larger. Hertwig has shewn that the germinal spot is often composed of two constituents as in the above cases, but that the more highly refractive material is generally completely enclosed by the less dense substance. By Fol the germinal spot is stated to be absent in a species of Sagitta, but this must be regarded as doubtful. In young ova the relative size of the germinal vesicle is very considerable. It occupies in the first instance a central position in the ovum, but at maturity is almost always found in close proximity to the surface. Its change of position in a large number of instances is accomplished during the growth of the ovum in the ovary, but in other cases does not take place till the ovum has been laid. As the ovum attains maturity, important changes take place in the constitution of the germinal vesicle, which are described in the next chapter.