220 FERTILIZATION 



maturation (i.e., development) when normally fertilized (fig. 137), but they 

 behave differently during the normal fertilization process. In some eggs, such 

 as the egg of Chaetopterus, there is only a slight change within the egg cortex 

 during fertilization, whereas in the egg of the teleost fish, the egg of the frog, 

 or in the egg of the urochordate, Styela, marked cortical changes involving 

 mass movements of protoplasmic materials can be demonstrated. 



c. Results Obtained by the Work on Artificial Parthenogenesis 



The question naturally arises: What has the work on artificial activation 

 of the egg contributed to the solution of the problems involved in egg activa- 

 tion? It has not, of course, solved the problem, but it has contributed much 

 toward a better understanding of the processes concerned with egg activation 

 and of the general problem of growth stimulation including cell division. We 

 may summarize the contributions of this work as follows: 



( 1 ) It has demonstrated that the egg in its normal development reaches 

 a condition when a factor (or factors) inhibits further development. 

 That is, it becomes blocked in a developmental sense. 



(2) It has shown that this inhibited state may be overcome and develop- 

 ment initiated by appropriate types of treatment. 



(3) It has revealed that activation of the egg is possible only at the time 

 when normal fertilization occurs in the particular species. In other 

 words, activation is possible only when favorable conditions are de- 

 veloped in the egg — conditions which enable it to respond to the acti- 

 vating stimulus. 



(4) It has demonstrated that one of the primary conditions necessary for 

 the initiation of division or cleavage of the egg is an initial increase 

 in the viscosity of the egg's cytoplasm. 



(5) Certain experiments suggest that chemical compounds, such as heparin 

 or heparin-like substances, may suppress cleavage and cell division, 

 presumably due to their ability to decrease the viscosity of the egg. 



(6) It therefore follows that substances and conditions which tend to in- 

 crease the egg's viscosity tend to overcome the inhibited state referred 

 to in ( 1 ) above and thus initiate development. 



(7) Recent evidence suggests that substances which produce cell injury 

 tend to initiate cell division in the egg. As states of injury have been 

 shown to produce growths of various kinds during embryonic devel- 

 opment and also during the post-embryonic period, it is probable that 

 the principles involved in egg activation are similar to those which 

 cause differentiation and growth in general. 



(8) A common factor, therefore, involved in egg stimulation and other 

 types of growths, including tumor-like growths, is the liberation of 

 some substance in the egg or in a cell which overcomes an inhibiting 



