PAR THENOGENESIS 



capable of developmental response to experimental means 

 are distributed among all of the four categories made above 

 with respect to the stage in the maturation process, in which 

 animal eggs are physiologically ripe for fertilization. 



Eggs which are fertilizable in the stage of first matura- 

 tion are more widespread in occurrence among animals 

 than those of any other class. Since both the facultative 

 and the fixed parthenogenetic eggs belong to this large 

 class, we might expect that experimental parthenogenesis 

 is more easily elicited in this class of eggs than in any other. 

 This expectation, however, is not fulfilled. Although the 

 Qg% of the starfish, fertilizable in the stage of first matura- 

 tion, and the first in which was discovered the capacity of 

 eggs to respond to experimental treatment with develop- 

 ment, is of all animal eggs the most readily responsive to 

 experimental means for parthenogenesis, other eggs of the 

 same class, as we shall learn, may fail wholly to respond 

 to means effective upon the starfish egg or they respond 

 with extremely abnormal development. Also, the greatest 

 number of positive results has been obtained on completely 

 matured eggs, i.e., those of sea-urchins. Finally, eggs of 

 the two remaining classes when properly treated develop 

 normally; frogs' eggs, for example, reach the adult stage. 

 We can not therefore correlate the ease with which experi- 

 mental treatment elicits parthenogenesis with the nuclear 

 state of the egg. Whether or not the restriction of natural 

 parthenogenesis to eggs fertilizable in the stage of first 

 maturation bears a causal relation to this mitotic phase of 

 the nucleus, has not yet been determined. As to the cause 

 of experimental parthenogenesis, however, there are only 

 two possibilities: it rests either in the egg's cytoplasm or 

 in the means that brings about the parthenogenetic 

 development. 



The latter view one often meets, namely, that the means 

 carries into the egg something which initiates its develop- 

 ment. The brief review of the response of individual eggs 



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