mixed under the microscope, and all the phenomena of the maturation of the egg and its impregnation can then be followed as well as the processes of segmentation which result in the conversion of the single-celled egg into the many-celled embryo. Interesting as a description of these phenomena would be, we must pass them by, for we have not yet described one of the most important processes of study.
Studying embryos, even the most transparent ones, by simply watching them under the microscope, leaves many features of the method of formation of the internal structure unknown, while in the case of opaque forms it reveals not a single feature except those of the surface. A knowledge of these internal points is, however, just as important as of the external modifications of form. In many, yes, in almost every case, the embryo is too small to be dissected, but by converting it into a series of slices or sections, and then studying these, structures and processes of growth are revealed which otherwise would remain entirely unknown. This method of section-cutting and the processes of preserving the sections thus obtained is almost entirely a growth of the last ten years. It is true that for a long time naturalists have resorted to it, but so crude were the instruments and so faulty the technique that section-cutting could hardly be said to exist in comparison with its importance to-day.
Here, as elsewhere, details would be out of place in an article of this character, but an outline of the processes involved in section-cutting will show the capacities of modern research as well as the methods which our student must master before he can take his place among the advanced workers of to-day. It must be said, in passing, that for every form some process is best adapted, and that what works well for one is often utterly unsuited for a closely related species. No general rule can be laid down by which the student can at once say that such and such methods are best adapted to give good results; the exact course of procedure in any case can only be determined by experiment.
Were it attempted to cut the fresh egg into sections, the result would be an ignominious failure. There are various preparatory processes necessary, and in all of these care must be exercised that the reagents employed do not produce abnormal effects. First, the egg must be hardened, and here there is a choice among a number of chemicals—alcohol, chromic acid, bichromate of potash, osmic or nitric acids, corrosive sublimate, etc.—each of which has its especial advantages and disadvantages. Even in the method of killing the egg previous to hardening, there are a number of methods to choose from. The hardening reagents all serve to kill, but not equally well, for they do not all work with the same rapidity. In the use of all, care has to be exercised to prevent contraction.
"Were we to cut the hardened egg, our sections, without further treatment, would reveal but little, for they would be very transparent,
