PROCESS OF REPRODUCTION IN ORGANISMS. l"J 



indicative of a low rate of metabolism. Haecker, for example 

 has induced the appearance of paired chromosomes in somatic 

 cells by the use of ether, and various other cases of the appearace 

 of tetrads and heterotypic mitoses in somatic cells have been re- 

 corded, in some of which the phenomena appeal to be associated 

 with conditions which decrease metabolism. According to this 

 view then the peculiar characteristics of the maturation divisions 

 are connected with the physiological condition of the gamete. 

 The stimulus to these divisions is apparently in many cases 

 the change in environment, physiological or physical, consequent 

 upon complete isolation from the parent body. In many other 

 cases, however, this stimulus is apparently not sufficient to bring 

 about maturation or its completion and this does not occur until 

 the sperm enters the egg. 



Turning now to the spermatozoon, we find that its history is 

 differenc from that of the egg. Here there is no such enormous 

 increase in volume as in the egg, but maturation divisions of the 

 same type as in the egg occur. In the later stages of the develop- 

 ment of the spermatozoon the cytoplasm of the sperm cell is in 

 part used up in metabolism and in part converted into stable 

 structural substance. The nuclear substance of the spermato- 

 zoon is evidently in a very different condition from that of the 

 egg. The sperm nucleus is separated from the external world 

 only by thin membranes. There is no evidence that the sperma- 

 tozoon, even in a nutritive fluid, can make new cytoplasm and 

 return to the condition of an ordinary cell. Apparently it is 

 irrevocably committed to the function of converting other forms 

 of energy into energy of locomotion. In short, the spermatozoon 

 is apparently also approaching death. Its life may be prolonged 

 by certain conditions or by quiescence for a considerable period, 

 but sooner or later it dies unless it enters the cytoplasm of an egg. 



But shall we call the spermatozoon an old or a young cell? 

 According to Minot's criterion it must be a very young cell, since 

 it consists very largely of nuclear material. We know but little 

 concerning the rate of metabolism in the ripe spermatozoon as 

 compared with earlier stages, but in all probability it is low. 

 The energy necessary for movement is undoubtedly slight and 

 it is not at all improbable that this energy may be furnished by 



