[13] PEESERVATION OF MICROSCOPIC MATERIALS. 619 



by Canada balsam as early as possible. The same method is adapted 

 to other microscopic animals." 



In order to confine infusorians or very small embryos, so as to sub- 

 ject them to the foregoing treatment under a cover glass, a method 

 which has given ^'ery good satisfaction is the following : Three very 

 small pellets of bees-wax or bees-wax and olive oil are stuck to the one 

 side of a cover near its margin, and arranged so as to form the angles 

 of a triangle. The drop containing the organisms is then placed on the 

 slide and the cover with its wax feet carefully laid upon it. A needle 

 is then taken and its point pressed down upon the cover glass until it 

 is forced downwards on its yielding supports and until its tfnder surface 

 is sufficiently approximated to the upper surface of the slide to clamp 

 the living infusorians or embryos fast which have been placed between 

 without actual compression. The application of the reagents may then 

 be leisurely proceeded with without washing the objects away, as one 

 reagent after the other is applied at one side of the cover and absorbed 

 on the other by bits of bibulous paper. 



Hertwig's method of preiniring and cutting ampMMan eggs. — Whitman 

 has given the following condensed account of Hertwig's plan of dealing 

 with amphibian ova, which would doubtless give advantageous results 

 if applied to the treatment of fish ova which contain coarse yolk gran- 

 ules, which are apt to become detached from the sections in the process 

 of mounting when the whole embryo has first been saturated with par- 

 affine, which must be removed from the sections with warm turpentine 

 or chloroform, or with cold benzole or xylol. The two first are the solv- 

 ents which act most quickly when used for the purpose of removing 

 the paraffine : 



" Although the amphibian egg has long been a favorite object of study 

 among embryologists — and quite as much so since section-cutting came 

 into vogue as before — comparatively little progress has been made in 

 overcoming the difficulties that attend its preparation for the micro- 

 tome. The chief difficulties are found in freeing the egg from its gela- 

 tinous envelope, and in preparing it so as to avoid brittleness. 



" The best method that has thus far been proposed for these eggs is 

 unquestionably that of O. Hertwig, and I shall therefore give it in de- 

 tail. 



"1. In order to facilitate the removal of the gelatinous envelope the 

 eggs are placed in water heated almost to boiling (90-96° C.) for five 

 to ten minutes. The eggs are thus coagulated and somewhat hardened, 

 while the envelope separates a little from the surface of the egg and be- 

 comes more brittle. The envelope is then cut under water with sharp 

 scissors, and the egg shaken out through the rupture. With a little 

 experience a single cut suffices to free the egg. 



" 2. By the aid of a glass tube the egg is taken up and transferred 

 to chromic acid (one-half per cent.), or to alcohol, 70, 80, and 90 per 

 cent. Chromic acid renders the egg brittle, and the more so the longer 



