ONTOGENESIS 171 



Like a wound clock (to vary Preyer's comparison) , 

 the egg is a mass of matter, of which the parts, 

 although in unstable equilibrium, are at rest because 

 of a sort of inertia. When an appropriate " stimulus " 

 comes, whether that of the sperm or that of some 

 chemical or physical agent, cell division begins to 

 follow cell division in rapid succession. " Stim- 

 ulus " is used, of course, in a very broad sense, 

 since the changes induced in the egg-cytoplasm by 

 the sperm or by external agents are fundamental in 

 character and profoundly alter the chemical and 

 physical nature of the protoplasm. Like chemical 

 reactions in general, the rate of division is influ- 

 enced by the temperature. A regular rhythm of 

 O-production and CO 2 -production has been demon- 

 strated in cleaving eggs, which probably is an external 

 indication of the growth of the chromatin from the 

 cytoplasm (by oxidation), in the resting stage between 

 one mitosis and the next. 



Doubtless in both naturally parthenogenetic eggs 

 and ordinary eggs much the same sort of specific 

 stimulus is necessary to inaugurate development. 

 The parthenogenetic individual, however, is able to 

 supply that stimulus itself, whereas in the majority 

 of animals it must be supplied from without. 



Alternation of Generations in Animals. In the 

 worms that reproduce by fission, after a number of 



from the combination of two gametes of diverse origin must bring about 

 a very different end-result from that which would be the fate of the egg 

 if it were to develop by itself, i.e. it insures a bi-parental inheritance 

 (see Chapter VII). 



