REJUVENESCENCE IN EMBRYO AND LAR\A 411 



aroused only by experimental conditions which approximate more 

 closely the conditions of fertilization, and still others only by 

 fertilization itself, or conditions essentially identical with it. More- 

 over, it is by no means certain that the conditions concerned in 

 fertilization are exactly the same in all cases. The morphological 

 dilTerences in the gametes of different species show clearly enough 

 that the course of gametic development is not always the same, 

 and the assumption that the action of the sperm is always the same 

 seems to be unjustified. The result is, of course, essentially similar 

 in all cases, i.e., increased metabolic activity, transformation of 

 nutritive substances, and cell division, but different factors or 

 combinations of factors may be concerned in producing it in differ- 

 ent cases. The differences in the reaction of different eggs to the 

 experimental parthenogenic agents suggest that various degrees of 

 specialization exist in the process of fertihzation itself. The con- 

 ception of the gametes as highly speciaHzed, physiologically old 

 cells places the whole problem of the initiation of development by 

 either experimental or natural means in a new light. 



OXYGEN CONSUMPTION AND HEAT PRODUCTION DURING EARLY 



STAGES OF DEVELOPMENT 



The first stage of development is a period of repeated cell 

 division, the cleavage period, during which the proportion of 

 active cytoplasm and nuclear substance increases at the expense 

 of substances which were accumulated in the egg during its growth 

 and have been previously inactive; or in some organisms, where 

 the egg itself contains but little nutritive material, it becomes 

 dependent at an early stage on nutriment from without. 



Authorities are generally agreed that during at least some part 

 of this period an acceleration in the rate of metabolism occurs.' 

 According to Warburg and Loeb and Wasteneys the ox>'gen con- 

 sumption of sea-urchin eggs increases during the course of cleavage. 

 In the egg of the mollusk Aplysia limacina Buglia found that the 

 oxygen consumption decreased slightly while carbon-dioxide pro- 

 duction remained uniform during the earhest stages of cleavage, 

 but in later embryonic stages both underwent a marked increase 



' Buglia, 'oS; Loeb and Wasteneys, '11; Meyerhof, '11; Warburg, 'oS, '10. 



