442 suiniARY OF current researches relating to 



/3. Collecting, Mounting and Examining Objects, &c. 



Preparing and Cutting Amphibian Eggs.* — Althougli the 

 amphibian egg has long been a favourite 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 

 gelatinous envelope, and preparing it so as to avoid brittleness. 



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

 unquestionably that of 0. Hertwig. and Dr. C. 0. Whitman | therefore 

 gives it in detail 



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 

 6-10 minutes. The eggs are thus coagulated and somewhat hardened, 

 while the envelope separates a little from the sui-face of the egg and 

 becomes 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 (i per cent.), or to alcohol of 70. 80, and 90 per cent. 

 Chromic acid renders the egg brittle, and the more so the longer it 

 acts ; therefore the eggs should not be allowed to remain in it more 

 than twelve houi*s. While eggs hardened in chromic acid never change 

 their form or become soft when transferred to water, those hardened 

 in alcohol, when placed in water or very dilute alcohol, lose their 

 hardness, swell up, and often suffer changes in form. 



3. Alcoholic preparations are easily stained, but chromic acid 

 preparations are stained with such difficulty and so imperfectly that 

 Hertwig omitted it altogether. 



There is an important difference between alcohol and chromic acid 

 in their effect on the pigment of the egg. Chromic acid destroys the 

 pigment to some extent, and thus obliterates, or at least diminishes, 

 the contrast between pigmented and non-pigmented cell-layers. As 

 the distribution of the pigment is of considerable importance in the 

 study of the germ-lamellfe. it is well to supplement preparations in 

 chromic acid with those in alcohol, in which the pigment remains 

 undisturbed. 



4. Eggs hardened in chromic acid were imbedded almost exclu- 

 sively in the egg-mass recommended by Calberla. The great advan- 

 tage offered by this mass is, that it supplies a sort of antidote to the 

 brittleness of the egg. It glues the cell-layers together, so that the 

 thinnest sections can be obtained without danger of breaking. 



5. As the dorsal and ventral surfaces, and the fore and hind ends 

 can be recognized in very early stages, it is important to know pre- 

 cisely how the egg lies in the egg-mass in order to determine the 



* Jen. Zeitschr. f. NaturwUs., is. (1882) p. 249. 

 t Amer. Natural., xvii. (1883) pp. 272-4. 



