PROCESS OF REPRODUCTION IN ORGANISMS. 2$ 



process of rejuvenescence, while in the case of the gamete a 

 further stimulus is usually necessary. 



4. Natural and Artificial Parthenogenesis. 



In some cases the development of the egg into a new organism 

 begins at once when it is isolated from the parent organism 

 and without fertilization. How shall we account for these cases 

 of natural parthenogenesis? The behavior of parthenogenetic 

 eggs forces us, I believe, to the conclusion that they are cells 

 which, although visibly differentiated in the same direction as 

 the true gametes, are nevertheless not so highly differentiated 

 nor so old physiologically as eggs requiring fertilization. They 

 resemble asexual reproductive cells or cell masses in that the 

 stimulus resulting from physiological isolation from the parent 

 organism is sufficient to initiate the process of dedifferentiation 

 and rejuvenescence. 



In this connection it is of interest to note that in cases where 

 a single individual produces both parthenogenetic eggs and eggs 

 requiring fertilization, the parthenogenetic eggs are produced 

 earlier in the life of the parent organism than the eggs requiring 

 fertilization. In other words, the eggs produced at earlier stages 

 are, like other parts of the organism, not so highly differentiated 

 nor so old physiologically as those produced in later stages. 

 This fact of the relation of parthenogenetic and non-partheno- 

 genetic eggs to the life cycle of the parent organism seems to me 

 to constitute one of the strongest arguments in support of the 

 view that the gametes are physiologically integral parts of the 

 soma and that they differentiate and grow old in the same manner 

 as other parts; moreover, I know of no other way of accounting 

 for the fact. 



The eggs of some species, if left for a time without fertilization, 

 often show the beginnings of more or less normal development, 

 but sooner or later die. Such eggs are intermediate between the 

 true parthenogenetic and non-parthenogenetic eggs. They react 

 to the stimulus of isolation by a slightly increased metabolism 

 but the reaction is either insufficient to establish the processes 

 so that they continue, or else the stimulus of isolation is not 

 entirely adequate to initiate processes which are in all respects 

 of normal character. 



