360 V. THE NATURE OF EVOLUTION 



for this enzyme. The treatment with chloroform or the addition of 

 bacteria or starch paste are sufficient to activate the enzyme. More 

 recently activators from tissue and serum have been recorded. The 

 most rapid and potent activators have been prepared from bacterial 

 sources, notably the so-called streptokinase from streptococcal culture 

 filtrates, and staphylokinase from staphylococcal cultures. These ac- 

 tivators may be comparable to a sperm or artificial stimuli that can 

 initiate the development of the egg cell ; the action of all these agents 

 consists in the faculty of disturbing the protein structure. 



The sperm cell may thus initiate the development of the egg cell 

 as a mere stimulus, but besides this the sperm will play a decisive 

 role in the establishm.ent of rejuvenescence as already discussed in 

 Part III. It must be desirable for the rejuvenescence that the sperm 

 is as different as possible in structure from the egg. This may be the 

 reason why hybrids are generally robust. However, two protein mole- 

 cules cannot combine if their structures are entirely different from 

 each other, so that for the establishment of the fertilization the sperm 

 cannot be different from the egg too extensively. The development 

 of the egg of a species of frogs or newts may sometimes be initiated 

 by the sperm of some other species, but the embryos will perish earlier 

 or later (34), presumably resulting from the difficulty of the mutual 

 fusion on account of the too different structures. 



