16 BIOLOGY 



of the semi-permeability of the surface films of living protoplasm 

 made it possible to recognize the variable which determines the ex- 

 change of liquids between protoplasm and the liquid medium by 

 which it is surrounded, namely, the osmotic pressure. Inasmuch 

 as the osmotic pressure is measurable, this field of biology has en- 

 tered upon a stage where every hypothesis can be tested exactly, 

 and biology is no longer compelled to carry a ballast of shallow 

 phrases. We are now able to analyze quantitatively such functions 

 as lymph formation and the secretion of glands. 



Recent investigations have thrown some light on the nature of 

 the conditions which seem to determine the semi-permeability of 

 living matter. Quincke had already mentioned that a film of oil 

 acts like a semi-permeable membrane. From certain considerations 

 of surface tension and surface energy it follows that every particle 

 of protoplasm which is surrounded by a watery liquid must form 

 an extremely thin film of oil at its surface. Overton has recently 

 shown that of all dissolved substances those which possess a high 

 solubility in fat, e. g., alcohol, ether, chloroform, diffuse most easily 

 into living cells. Overton concludes that lipoid substances, such as 

 lecithin and cholesterin, which are found in every cell, determine the 

 phenomenon of the semi-permeability of living matter. 



IV. Development and Heredity 



We now come to the discussion of those phenomena which con- 

 stitute the specific difference between living machines and the ma- 

 chines which we have thus far been able to make artificially. Living 

 organisms show the phenomena of development. During the last 

 century it was ascertained that the development of an animal egg, 

 in general, does not occur until a spermatozoon has entered it, but, 

 as already stated, we do not know which variable in the egg is changed 

 by the spermatozoon. An attempt has been made to fill the gap by 

 causing unfertilized eggs to develop with the aid of physicochemical 

 means. The decisive variable by which such an artificial partheno- 

 genesis can be best produced is the osmotic pressure. It has been 

 possible to cause the unfertilized eggs of echinoderms, annelids, 

 and mollusks to develop into swimming larvas by increasing transi- 

 torily the osmotic pressure of the surrounding solution. Even in 

 vertebrates (the frog and petromyzon), Bataillon has succeeded in 

 calling forth the first processes of development in this way. In 

 other forms specific chemical influences cause the development, 

 e. g., in the eggs of starfish diluted acids, and, best of all, as De- 

 lage has shown, carbon dioxide. In the eggs of Chcetopterus potas- 

 sium salts produce this result, and in the case of Arnphitrite, calcium 

 salts. 



