THE PERMEABILITY OP ORGANIC MEMBRANES 139 



tain the weight of its own contents, for on draining off 

 the water very slowly rupture always occurs. Before 

 this osmotic intake of liquid, the yolk membrane is, if 

 the egg is fresh, quite able to remain on a flat surface un- 

 supported by surrounding liquid. 



Osmotic and imbibitional absorption of water may also 

 be illustrated by the following experiment : The shell of 

 an egg is dissolved away completely by dilute hydro- 

 chloric or acetic acid, which penetrates slowly into the 

 interior. When placed in pure water the egg increases in 

 volume to a very marked degree, the membrane becoming 

 tense. In a strong salt or sugar solution the volume 

 diminishes only slightly, partly because the membrane 

 is by no means strictly semi-permeable, and partly because 

 much of the increase is due to the imbibitional swelling of 

 the contents of the egg, which have been coagulated by the 

 acid. 



It may be remarked that for demonstration purposes 

 semi-permeable membranes of collodion mounted on silver 

 gauze, and on which copper ferrocyanide has been pre- 

 cipitated, seem very suitable and easy to prepare. Fouard 

 (1911), who introduced the method quite recently, has even 

 used it for quantitative work, such as molecular weight 

 determinations. 



PERMEABILITY OF SEED COATS. 



While engaged in an investigation on barley seeds, 

 A. J. Brown (1909) found that the seed coat exhibited a 

 very peculiar selective permeability, in that whereas 

 water passed through it freely, and ionized salts, sugars, 

 and other substances, were held back, yet certain feebly 

 ionized salts, such as mercuric chloride and cadmium iodide, 

 together with alcohols, aldehydes, ketones, esters, and 

 the simple fatty acids, were able to enter the seeds more or 



