=Development of the Fertilised Ovum.=--The first stages in the development of the Mollusca are identical with those which occur in other classes of animals. The fertilised ovum consists of a vitellus or yolk, which is surrounded with albumen, and is either contained in a separate capsule, or else several, sometimes many, ova are found in the same capsule, only a small proportion of which ultimately develop. The germinal vesicle, which is situated at one side of the vitellus, undergoes unequal segmentation, the result of which is usually the formation of a layer of small ectoderm cells overlying a few much larger cells which contain nearly the whole of the yolk. The large cells are then invaginated, or are simply covered by the growth of the ectoderm cells. The result in either case is the formation of an area, the blastopore, where the inner cells are not covered by the ectoderm. The blastopore gradually narrows to a circular opening, which, in the great majority of cases, eventually becomes the mouth. The usual differentiation of germinal layers takes place, the epiblast eventually giving rise to the epidermis, nervous system, and special sense organs, the hypoblast to the liver and to the middle region of the alimentary tract, the mesoblast to the muscles, the body cavity, the vascular, the excretory and reproductive systems. The next, or _trochosphere_ (_trochophora_) stage, involves the formation of a circlet of praeoral cilia, dividing the still nearly spherical embryo into two unequal portions, the smaller of which consists simply of the prostomium, or part in front of the mouth, the larger bearing the mouth and anus. So far the series of changes undergone by the embryo are not peculiar to the Mollusca; we now come to those which are definitely characteristic of that group. The stage next succeeding the development of the trochosphere is the definitive formation of the _velum_, a process especially characteristic of the Gasteropoda and Pelecypoda, but apparently not occurring in the great majority of land Pulmonata. [Illustration: FIG. 44.--Veligers of _Dentalium entalis_ L.: =A=, longitudinal section of a larva 14 hours old, × 285; =B=, larva of 37 hours, × 165; =C=, longitudinal section of larva of 34 hours, × 165; _m_, mouth; _v_, _v_, velum. (After Kowalewsky.)] The circlet of cilia becomes pushed more and more towards the anterior portion of the embryo, the cilia themselves become longer, while the portion of the body from which they spring becomes elevated into a ridge or ring, which, as a rule, develops on each side a more or less pronounced lobe. The name _velum_ is applied to this entire process of ciliated ring and lobes, and to the area which they enclose. [Illustration: FIG.45.--Veliger of _Patella vulgata_ L., 130 hours old: _f_, rudimentary foot; _op_, operculum; _sh_, shell; _v_, _v_, velum. (After Patten, highly magnified.)]