314 THE PHYSIOLOGY OF REPRODUCTION 



unfertilised eggs of a sea-urchin (Arbacia), that such eggs, when 

 exposed for two hours to a mixture of equal parts of sea- water and 

 of a 10/8 m. MgCl. 2 solution and then replaced in normal sea- water, 

 developed into swimming larv;e. The effect of adding the MgCL, 

 solution was to raise the osmotic pressure considerably, and further 

 experiments showed that this increase of osmotic pressure was the 

 essential factor in this original method of artificial parthenogenesis. 

 It had thus become possible to replace the mysterious complex 

 " living spermatozoon " by well-known physico-chemical agencies. He 

 found subsequently that a great variety of different agencies are able 

 to induce parthenogenesis. From a wealth of experimental data Loeb 

 has evolved a theory which may be summarised as follows : The 

 development of the mature ovum is dependent upon two processes. 

 It is initiated by a cytolysis affecting the periphery of the cell. 

 Morphologically this process is represented by the formation of a 

 fertilisation membrane, which is really the essential step in the 

 activation of development. Membrane formation can be induced 

 by any agent having a cytolytic action : fatty acids, saponin and 

 similar substances, lipoid solvents, bases, rise in temperature, certain 

 inorganic salts, e.g. BaCl 2 , CaCl 2 , KCNS, and lastly, the blood-serum 

 or cell extracts of certain species, but foreign to the species to which 

 the ovum belongs. 



The next question is : Why is a cytolysis necessary to initiate 

 the development of the egg? The answer is to be found in the 

 observations of Warburg, 1 that the oxygen consumption of the sea- 

 urchin egg shows a sudden increase after fertilisation to seven times 

 that of the unfertilised egg. Mechanical disintegration of the 

 unfertilised egg, even injury to the cortical layer, results in a similar 

 increase in oxygen consumption. These results were confirmed by 

 Loeb and Wasteneys for the eggs of Strongylocentrotus purpuratus. 

 They also found an increased oxygen consumption after the action of 

 cytolytic agents and after the formation of a fertilisation membrane. 

 It would follow that the contents of the unfertilised egg are in 

 a highly oxidisable condition, but that the cortical layer of such an 

 egg does not present conditions favourable for the oxidation of its 

 contents to take place. Cytolysis removes an obstacle so that 

 the oxidations become possible. Loeb suggests that by breaking 

 down the peripheral lipoid emulsion, certain substances which were 

 previously solid are liquefied and allowed to diffuse into the 

 egg, where they start or accelerate the chemical processes under- 

 lying development. The oxygen consumption increases before cell 



1 Warburg, " Beitrage zur Physiologic der Zelle, insbesondere iiber die 

 Oxydations gtfschwindigkeit in Zellen," Ergebnisse der Physivlogie, 1914, vol. xiv. ; 

 see also Zeitsch. f. physiol. Chem., vol. Ivii., 1908 ; vol. lx., 1909 ; vol. Ixvi., 1910. 



