EXAMINATION AND PRESERVATION MEDIA 219 



destroyed b\- its action if prolonged (for instance, red-blood 

 corpuscles). 



In order to render it inoffensive to fresh tissues it must have dissolved 

 in it substances of similar diffusibility to those of the liquids of the 

 tissue, so as to prevent the occurrence of osmosis, to which process the 

 destructive action of pure water is mainly due. Now cell contents are 

 a mixture of colloids and crystalloids ; consequently, in order to reduce 

 osmotic processes to a minimum, it is necessary that the examination 

 medium contain a due proportion of both crystalloids and colloids. By 

 adding, for instance, white of egg to salt solution this end may be in 

 some measure attained ; and, as a matter of fact, the liquids recom- 

 mended as " indifferent " are generally found to contain both crystalloids 

 and colloids. Liquids thus composed, in which tissue-elements are in 

 osmotic equilibrium — that is, neither swell nor shrink — are said to be 

 isotonic to the tissues ; whilst those in which they shrink are called 

 hypertonic, and those in which they swell hypotonic. Solutions of 

 common salt, in different concentrations, form the base of the most 

 commonly employed isotonic liquids. For marine Invertebrates, sea- 

 water is generally isotonic. 



441. Salt Solution. See Addendum, p. 731, § 1430, for various 

 useful media. 



442. Pictet's Liquid {Mitth. Zool. Stat. Neapel, x, 1891, p. 89). 

 Five to ten per cent, solution of chloride of manganese. These 

 proportions are for marine animals, and for terrestrial animals 

 will generally be found much too high. For these from 1 to 3 per 

 cent, will be nearer the mark. Lee found this liquid excellent. 



443. Aqueous Humour, Simple White of Egg. Require no 

 preparation beyond filtering. They may be iodised if desired 

 (see next §), or mixed with salt solution. 



444. Iodised Serum. Max Schultze (Virchow's Archiv., xxx, 

 1864, p. 263). We take the following from Ranvier (Traite, 

 p. 76. 



The only serum that gives really good results is the amniotic liquid 

 of mammals. Flakes of iodine are added to it, and the flask frequently 

 agitated for some days. The flask should have a wide bottom, so that 

 the serum may form only a shallow layer in it. 



Another method is as follows : Serum is mixed with a large pro- 

 portion of tincture of iodine ; the precipitate that forms is removed 

 by filtration, and there remains a strong solution of iodine in serum. 

 This should be kept in stock, and a little of it added every two or three 

 days to the serum that is intended for use. In general for maceration 

 purposes a serum of a pale brown colour should be employed. 



445. Artificial Iodised Serum (Frey, Das Mikroskop, 6 Aufl., 

 1877, p. 75). Distilled water 270 grm., white of egg 30, sodium 

 chloride 2-5. Mix, filter, and add tincture of iodine. 



446. Migula's Glycerised Blood-serum (see the paper in Zeit.f. iviss. 

 Mik., vii, 2, 1890, p. 172). 



447. Chloride of Calcium (Harting, Das Mikroskop, 2 Aufl., p. 297). 

 The aqueous solution, either saturated or diluted with 4 to 8 parts of 

 water, has a low refractive index and does not dry up. 



