THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 145 



Atmospheric pressure. — The effect of variations in atmospheric pressure on 

 mitochondria has not been studied on account of the technical difficulties; for this 

 reason a comparison of their relations in alpine and deep-sea fauna might yield 

 interesting results. 



Osmotic pressure. — The effect of variations in osmotic pressure on mitochondria 

 might be studied by comparing the mitochondria in fresh-water forms with those 

 inhabiting the most concentrated brine. In plants it is not difficult to find a great 

 variation in osmotic pressure. 



Acidity. — One of the most characteristic properties of mitochondria is their 

 solubility in mixtures containing even a small amount of acetic acid (0.5 to 2.5 

 per cent). It would be interesting to study their relations in vinegar eels, which 

 normally five in an environment containing 4 per cent of acetic acid. The sali- 

 vary glands of the rock-boring mollusc Dolium galea, which secretes sulphuric acid 

 in a concentration of 4 or 5 per cent (see Bayliss, 1915, p. 359), and the acid-forming 

 cells of plants would also repay investigation. If Macallum (1908, p. 628) and 

 others are right (which seems, however, unlikely) in assuming that the hydro- 

 chloric acid of the gastric juice is formed in the parietal cells, then these cells must 

 contain hydrochloric acid in much higher concentration than the juice (0.3 to 0.45 

 per cent), and Regaud's (1908a, p. 18) assertion that they are devoid of mitochon- 

 dria becomes of vital importance. 



Water-content. — The effect of variations in the water-content upon the form of 

 mitochondria has never been determined. For this purpose the Scyphozoa, with a 

 water-content of 99 per cent, and the Trochebninthes, which can survive prolonged 

 desiccation (Parker and Haswell, 1897, p. 309), offer an excellent opportunity. 



Hibernating animals take no water, though they continue to excrete urine. 

 All of the water which is absolutely necessary for the continuance of their vital 

 processes is metabolic, being produced by oxidation of the proteins, fats, and carbo- 

 hydrates of the tissues through respiration (Babcock, 1912, p. 170). The general 

 slowing-up of metabolism, the drowsiness and sleepiness, is in all {irobability due 

 to a reduced water-content, with consequent retardation of all chemical reactions. 

 The fact that the fats yield more water than either the proteins or the carbohydrates, 

 in the case of some fats even more than their own weight of water, would seem to 

 indicate the possibility of there being some change in the mitochondria (which 

 are, themselves, phosphoUpins containing glycerol and fatty acids among other 

 things). One would expect a diminution. The well-known occurrence of other 

 cytological variations in nerve-cells during hibernation would also indicate that a 

 study of mitochondria in this condition might give valuable results. Further- 

 more, since mitochondria are present in almost all the tissues of all animals, we 

 must entertain the possibihty that they may be in part the source of metabolic 

 water in general. For this reason investigations into their relations in the common 

 clothes moth, Tinea pellionella, desert animals like serpents and prairie-dogs, and 

 sea-birds which have no opportunity to drink fresh water, should be undertaken. 



Respiration. — Still more recently it has been claimed that the chief function 

 of mitochondria is protoplasmic respiration (p. 133). There seem to be several 

 ways by which this hypothesis can be tested: 



