48 EMBRYOLOGY 



make the egg a greater source of energy, and it may be that the increased 

 energy is needed for the physical and chemical processes in cell division, 

 gastrulation, and development in general. 



Along with the increased metabolism there is an increase in permeability 

 of the cell membrane. Substances get into the fertilized egg more easily than 

 into the unfertilized egg. This increased permeability may be correlated with 

 the increase in metabolism and may allow for freer exchange of substances 

 through this cell membrane. Another increase is in the amount of ammonia 

 excreted, and this is important because it indicates that the purine metabolism 

 of the egg is stimulated. Purine metabolism is important in the synthesis of 

 nuclear constituents. 



• There are a number of other chemical changes which can be correlated 

 with fertilization, but let us pass on to a physical change which occurs at 

 fertilization. This is an increase in the viscosity of the protoplasm. The 

 protoplasm becomes more solid, more like a gel. And this change appears to 

 be necessary for cell division. 



The above observations cover only some of the tremendous changes 

 which go on in the metabolism of the egg at the same time that the sperm is 

 stimulating it to develop. Indeed, the chemical changes at fertilization re- 

 semble closely those which occur when a muscle is stimulated to contract, 

 and it is tempting to consider stimulation of muscle and the stimulation of 

 the egg as very similar problems. 



Artificial parthenogenesis 



Now although the sperm produces all these rather profound changes 

 in the condition of the protoplasm of the egg, the same changes can be 

 brought about by artificial means — by treatment with a number of salt solu- 

 tions, acids, and other substances. Some of the changes which we have just 

 outlined above may also occur in artificial parthenogenesis, exactly as they 

 do when the sperm enters. If the sea urchin egg is put through a mixture of 

 sodium chloride and sea water for a few minutes and then returned to 

 normal sea water, the eggs undergo most of these changes, undergo cell 

 division, and go through the whole process of development. 



In the frog it is necessary only to prick the egg with a needle coated 

 with blood serum. Some of the eggs will develop into mature frogs; others 

 merely undergo some development and then die. The eggs of many species, 



