FERTILIZATION AFTER INITIATION OF DEVELOPMENT. 289 

 V. DISCUSSION. 



For an egg to develop, some stimulus must initiate certain 

 changes within it which, having been once set in motion, appear 

 to be autonomous. If an egg is not fertilized, or its development 

 initiated by some artificial agent, it normally does not develop. 

 But studies in the field of the artificial initiation of development 

 have brought to light many methods that may be employed 

 successfully to set in motion the developmental mechanics latent 

 within the egg. As yet we are unable to determine just the 

 mechanism by which these agents are effective, but practically 

 all methods agree in one essential at least the permeability of 

 the egg membrane is increased. 



Practically every one is agreed that the egg must be considered, 

 a very delicately adjusted physico-chemical system, within which, 

 if a spermatozoon gains entrance, a characteristic series of reac- 

 tions are set in motion that lead to development and a consequent 

 change of the system. If artificial agents are employed as a 

 substitute for the spermatozoon not only do we have conditions 

 changed but also if the proper procedure is known and applied 

 the end results are essentially the same as when initiation was 

 effected by a spermatozoon, e. g., cleavage, gastrulation and 

 swimming larvse. After the initiatory changes have been in- 

 duced a spermatozoon in either case is entirely unable to produce 

 any of its characteristic effects. 



The delicately adjusted condition of the egg is only a tem- 

 porary thing for if sea-urchin eggs are allowed to stand in sea- 

 water for a day or more they gradually lose their power of fer- 

 tilization. Not only can this unfertilizable condition, accom- 

 panied by increased permeability, be produced by standing but 

 also by hypertonic sea-water, butyric acid, by a slight rise in 

 temperature of the sea-water and several other agents including, 

 of course, fertilization. 



Many writers have considered an unfertilizable condition as 

 an indication of the death of the egg but to cite only one instance 

 to disprove this, the writer has called attention to the fact that 

 if initiation of development is partially accomplished by a treat- 

 ment with butyric acid and the membranes so formed are de- 

 stroyed by shaking, sperm will enter the eggs of Arbacia but do 



