258 CHROMATIN, ANIMAL CHROMOSOMES, NUCLEOLI 



some other way, directly in the mounting medium, and then placed 

 in a hanging drop, with a pipette, or mounted directly between a 

 clean slide and coverslip. In either case the coverslip must be 

 sealed with vaseline to prevent evaporation. Successful hanging 

 drop preparations may be good for as long as sixty hours, but the 

 simpler method gives normal cells for a maximum of twenty-four 

 hours. 



The mounting medium must be isotonic with the cells to be 

 studied. In hypotonic media the cells swell and undergo other 

 physical changes while in hypertonic solutions they shrink, 

 Belah suggests that the medium used for mounting be made 

 slightly hypotonic (by the addition of distilled water) since some 

 water is unavoidably lost in making preparations. The amount. of 

 dilution required is learned by experience. 



The control of the hydrogen ion concentration of the mounting 

 medium is an important element of success. This should be 7' 

 or very close to it. A slight alkalinity is less harmful than an 

 acid reaction. The _pH may be controlled either by the addition 

 of primary sodium phosphate (Belaf) or of carbon dioxide 

 (Sakamura). 



Another simple and useful method for studying living cells is to 

 mount the excised tissue on a clean slide, in a mixture of normal 

 body fluid and paraffin oil and then cover w^th a coverslip. For 

 studying in the living state in Chironomus. the giant chromosomes 

 which are found in the salivary glands, Bauer gives the following 

 directions {Zeitsch.f. Zellforsch. u. mikr. Anat., xxiii, 1935, p, 280) : 

 The glands are dissected out in the body fluid and then trans- 

 ferred to a clean slide, along with some of the body fluid, with a 

 pipette. Several large drops of paraffin oil are now added to the 

 tissue and the whole mass is covered with a coverslip. According 

 to Bauer, the cells of the salivary gland will live a number of 

 hours under these conditions. If the glands are taken from 

 small larvae; of other Diptera, Ringer's solution can be used in 

 place of the body fluids. For various Ringer solutions and 

 Baker's solution see § 1430. 



622. Study of Fresh and Lightly Fixed Cells. It is often desirable 

 to determine quickly the character of a tissue, the stage of an 

 embryo, the presence of mitotic figures of a certain stage, etc., 

 before more elaborate techniques are employed. For this purpose 

 the object to be studied can be placed in one of the indifferent 

 media, mechanically dissociated, if need be, mounted and examined 

 directly, or else stained with aceto-carmine or acidulated methyl 

 green. The medium most generally used is Ringer's solution, or 

 physiological salt solution, but there are a number of others, such 

 as serum, iodised serum, aqueous humour, lymph, or amniotic 

 fluid. Some of these seem to be somewhat toxic, but for the 

 purpose in hand they are extremely useful. 



