PROCESS OF REPRODUCTION IN ORGANISMS. 19, 



3. The Nature and Result of Fertilization. 



In experimental reproduction resulting from the physical 

 isolation of pieces and in asexual reproduction in nature the 

 physical or physiological isolation of the part is usually sufficient 

 to initiate the processes of dedifferentiation and rejuvenescence. 

 In the case of the gametes in animals, however, differentiation 

 of the cells has in most cases proceeded so far and the specialized 

 structure has become so firmly fixed that physical or physio- 

 logical isolation does not constitute a sufficient stimulus to 

 initiate dedifferentiation. The maturation divisions represent 

 perhaps an attempt of the cells to begin the process, but they 

 are ineffective since nuclear synthesis is no longer possible and 

 the cells continue to approach death, for the egg is unable to 

 use its accumulated substance as a source of energy and the 

 spermatozoon possesses no material or only a minimal amount 

 which can serve as a source of energy. 



But with the entrance of the spermatozoon into the egg condi- 

 tions are at once changed. The metabolism in the resulting 

 zygote in animals usually undergoes a rapid increase and the 

 structural material of the egg cytoplasm, which under previously 

 existing conditions remained inactive, now begins to enter 

 metabolism and to serve as a source of energy. The sperm 

 nucleus now finds itself surrounded by abundant nutritive ma- 

 terial under such conditions that it can be used in synthesis. 

 Moreover, as the cytoplasmic material of the egg is used up, 

 the obstacles to metabolism become less and less and the rate of 

 metabolism becomes higher and higher, i. e., rejuvenescence 

 proceeds at an increasingly rapid rate. In the sea urchin, as 

 Meyerhof ('n) has recently shown, the oxidation processes are 

 four times as great in amount in the larvae shortly after the 

 swimming stage is reached as in the first hour after fertilization. 

 In those cases where the zygote forms a "resting stage," either 

 the process of rejuvenescence goes on very slowly or else the 

 external conditions which lead to the formation of gametes 

 retard or inhibit the development at an early stage. 



Whether the primary effect of the entrance of the sperm is a 

 superficial cytolysis, as Loeb believes, or an increase in permea- 

 bility, as R. S. Lillie and others have suggested, the result is 



