636 THE POPULAR SCIENCE MONTHLY. 



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 Ave 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 other- 

 wise 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 pro- 

 cesses 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 advan- 

 tages and disadvantages. Even in the method of killing the egg pre- 

 vious 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, 



