Page:Popular Science Monthly Volume 30.djvu/657

Rh be found congregated at the lightest side of the dish. A lamp seems to exert the same fascination on them as on the moths of a summer's night. The student, armed with a magnifying-glass, now picks out from the dish the forms he desires to study by means of a medicine-dropper, and transfers them to a separate dish, where they may have an abundance of water, and, when sufficient material has been picked out, the real study begins.

The first thing is to ascertain everything of the external and internal structure that can be seen in the living animal. For this purpose it is placed on a glass slide in a drop of sea-water and carefully studied under the microscope. In this, as in all embryological work, drawing is absolutely necessary. Pages of description will not take the place of pictorial representation. After the whole is studied, then comes a study of the different parts, drawings and notes being made of each. An embryo is continually growing, and it becomes necessary to take into account every stage of growth. The embryo of to-morrow will be different from that of to-day, and the changes must be recorded. Some of the embryos are therefore kept, the water being changed, as often as necessary, and these serve for to-morrow's study, the drawings of to-day furnishing a basis of comparison. In many cases it is a comparatively easy task to rear embryos until the adult condition is recognizable, but at other times it is found impossible to keep them in confinement for more than two or three days. In the first case it is an easy task to identify the forms studied, but in the other the difficulty is considerable. Subsequent skimmings must be made in the hopes of securing the later stages of development, while an endeavor to find the animal which produces the eggs frequently meets with success. Comparison with the studies of other investigators is also an important aid to identification.

If, however, the eggs are taken directly from the parents, this trouble is wholly avoided, although other difficulties are introduced. Suppose, for instance, that one wishes to study the development of one of the fishes, the first step is to obtain males and females with the generative products ripe. A gentle stroking will serve to expel both eggs and milt, and then these are mixed together and "artificial impregnation" is affected. In the case of worms, oysters, and clams, the same result can be obtained by mincing the generative organs of ripe males and females, mixing them together, and then straining off the larger portions which, by their decay, would soon pollute the water. In the case of crabs and shrimps the eggs are borne attached to the abdominal legs of the mothers, and by capturing these females an abundant supply of material can be obtained. The parents can readily be preserved alive in lobster-cars or similar contrivances, and furnish eggs as they are needed.

Artificial impregnation is a very valuable process, for, by its aid, every stage in development may be obtained. Eggs and milt may be