REJUVEXESCE.\XE L\ EMBRYO AND LARVA 417 



heart-beat and hatching is probably due in pari to increased func- 

 tional activity and stimulation, but it may be largely the con- 

 sequence of the increasing lipoid content of the nervous system in 

 connection with medullation of the nerves, a change which would 

 increase the relative concentration of phenyl urethane in the 

 nervous system and so might intensify its action (see pp. 75-76). 

 In the vertebrates particularly these changes in the nervous system 

 make the use of the susceptibihty method with highly fat-soluble 

 substances difficult in the later stages of development. If this 

 second increase in susceptibility is due to the increase of fatty 

 substances in the nervous system, it of course does not mean that a 

 second period of rejuvenescence occurs, but rather that the sus- 

 ceptibility to phenyl urethane is not a measure of the metabolic 

 condition at this stage. In all probabiHty senescence and decrease 

 in metabolic rate continue from the stage where the susceptibility 

 first begins to decrease. 



In Tautogolabrus the period of increasing susceptibility con- 

 tinues up to the time of hatching, and almost all of the increase 

 occurs before movement or special function of organs begins. At 

 the periblast stage, where Fundulus shows the highest suscepti- 

 bility, Tautogolabrus has undergone only half of its increase and the 

 total increase of susceptibihty in the latter is about twice that 

 in the former. These differences between the two forms are 

 undoubtedly associated with differences in the course of develop- 

 ment. The second column of Tables XI and XII shows that 

 Tautogolabrus develops three or four times as rapidly as Fundulus, 

 and its development up to the time of hatching occurs very largely 

 at the expense of nutritive material in the protoplasmic part of the 

 egg, but little of the separate yolk mass being used during this 

 stage, while in Fundulus most of the yolk is used before hatching. 

 It is also evident that the protoplasms of the two species differ 

 widely in capacity for growth, for the egg of Fundulus is very much 

 larger and the adult usually much smaller than that of Tautogola- 

 brus. Apparently the differences between the two eggs determine 

 that the degree of rejuvenescence is much greater and that the 

 period of rejuvenescence extends to a much later stage of develop- 

 ment in Tautogolabrus than in Fundulus. 



