[17] PRESERVATION OF MICROSCOPIC MATERIALS. 623 



Hetmeguy^s method ofpreijarivig and mveMigating the eggs ofsalmonoids. 



" The ova of the Salmonido} are usually employed by embryologists in the 

 study of the development of the osseous fishes. It is difficult to examine 

 them in the fresh state, either whole or by transmitted light, on account 

 of the thickness of their envelopes, or after opening them, in consequence 

 of the small consistency of the germ, especially at the commencement 

 of segmentation. Chromic acid, the reagent most frequently employed 

 to harden these ova, readily alters the young cells, and deforms the 

 embryos by compressing them between the unextensible envelope of the 

 ovum and the solidified vitelline mass. For the last two years I have 

 employed in the laboratory of Comparative Embryology of the College 

 de France a process which enables us to extract the germs and embryos 

 from the ova of trout and salmon with the greatest facility, and without 

 causing them to undergo the least alteration. 



"I place the ovum for a few minutes m a 1 per cent, solution of osmic 

 acid until it has acquired a light brown color, then in a small vessel 

 containing MUller's fluid, and I open it with a fine pair of scissors in 

 the midst of this liquid. The central vitelline mass, which is coagulated 

 immediately on contact with water, dissolves, on the contrary, in the 

 Miiller's fluid, while the solidified germ and cortical layer may be ex- 

 tracted from the ovum and examined upon a glass plate. 



" By treating the germ with a solution of methyle greon and then 

 with glycerine I have been able to observe in the cells of segmentation 

 the very delicate phenomena lately indicated by Auerbach, Biitschli, 

 Strasburger, and Hertwig, and which accompany the division of the 

 nucleus, namely, the radiate arrangement of the protoplasm at the two 

 poles of the cell, the nuclear i)late, the bundles of filaments which start 

 from it, and the other succeeding phases. 



" This proves that the treatment undergone by the ovum does not at 

 all alter the elements of the germ. 



" In order to make cross-sections of the germs and embryos thus ex- 

 tracted from the ovum I leave them for some days in Miiller's liquid and 

 color theui with picrocarminate of ammonia. After depriving them of 

 water by treatment with alcohol of spec. grav. 0.828, and then with abso- 

 lute alcohol, I put them for twenty-four hours into collodion. The em- 

 bryo is then arranged upon a small slab of elder-pith soaked with alcohol, 

 and is covered with a layer of collodion. When the collodion has ar- 

 rived at a suitable consistency very thin sections may be made, includ- 

 ing the embryo and the plate of pith, and these are to be mounted and 

 preserved in glycerine. 



" This process is applicable to all sorts of embryos which are not very 

 thick, so that they may be colored en masse. It has the immense ad- 

 vantage of enabling one to see at what level in the embryo each section 

 is made, to preserve each section in the midst of a transparent mass, 

 whicb jsijstaips all the parts and prevents their being damaged, a« too 



