CHAPTER XV 



REJUVENESCENCE IN EMBRYONIC AND LARVAL 

 DEVELOPMENT 



If the gametes are physiologically old cells, rejuvenescence must 

 occur during embryonic development, for the organism when it 

 begins its active independent Ufe at the end of the embryonic 

 period is certainly very much younger in every respect than the 

 gametes before fertilization. It now remains to consider the 

 evidence bearing upon this point. This evidence is chiefly zoo- 

 logical rather than botanical, for in most of the plants the early 

 embryonic stages cannot readily be isolated for experimental 

 purposes. 



THE EFFECT OF FERTILIZATION 



To attempt any consideration of the problem of fertihzation 

 itself would lead us too far afield; moreover, no well-established 

 and generally accepted theory of fertilization has as yet emerged 

 from the great mass of often conflicting experimental data and 

 opinions. It is the effect of fertilization rather than the process 

 itself with which we are primarily concerned. 



Whatever the nature of the process, it is a self-evident fact that 

 the union of the two gametes is usually the starting-point of a 

 new period of activity and change in the resulting cell. It is true 

 that in some cases among both plants and animals fertihzation is 

 followed after a short period of activity by a quiescent period, but 

 we know that in certain of these cases the cessation of activity is 

 due to incidental or external factors, such as the presence of an 

 impermeable shell or envelope of some sort which cuts off the supply 

 of oxygen or water, or otherwise interferes with dynamic activity 

 until it is removed in one way or another, or gamete formation may 

 occur at seasons of the year or under external conditions which 

 retard or inhibit metabolic activity. In the delayed germination 

 of plant seeds,' in the quiescent encysted periods of certain protozoa 



' See, for example, Crocker, '06, '07, '09; and references to literature in these 

 papers. 



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