REJUVENESCENCE IN EMBRYO AND LARVA 415 



to the age cycle. In the earlier larval stages the animal is still 

 growing young, while in the later stages it is growing old. 



Between Nereis and another annelid, Arenicola cristata, an 

 interesting difference exists. During the period of rejuvenescence 

 the Nereis embryo obtains its nutritive material from the yolk in 

 the egg, but this material is used up before the end of the larval 

 period, and metamorphosis from the larval to the adult form does 

 not occur unless the larva can obtain food from without. The 

 egg of Arenicola, however, contains sufficient yolk to carry 

 development completely through the larval period and meta- 

 morphosis to the stage of a worm with five or six segments, after 

 which food from without is necessary. In both these forms the 

 embryonic period of increase in susceptibility, i.e., of rejuvenescence, 

 ends at about the stage when the last of the yolk is used up: the 

 Nereis embryo continues to grow younger only up to the larval 

 stage, while rejuvenescence in Arenicola continues through the 

 larval stage, the metamorphosis, and up to the six-segment stage 

 of the worm. During this period yolk is being transformed into 

 chemically active nuclear substance and cytoplasm, and the pro- 

 portion of chemically active to inactive substance increases to a 

 certain point where the accumulation of new structural substance, 

 together with any part of the old that may remain, balances the 

 synthesis of active protoplasm. 



Susceptibility determinations have been made for only two other 

 species of annelids, Chaetopterus pergamentaceus and Hydroides 

 dianthus, and in both rejuvenescence takes place during the embry- 

 onic period, as in Nereis, but the stage at which rejuvenescence 

 gives place to senescence was not determined in these forms. 



Among the vertebrates the eggs of two species of fish have been 

 used for susceptibility determinations. In contrast to the holo- 

 blastic egg of the starfish, sea-urchin, and annelid in which the 

 yolk is in all or some of the cells and the whole egg divides, the fish 

 eggs are meroblastic, most of the yolk being separated from the 

 active protoplasmic part of the egg, and only the latter divides. 

 In such eggs the embryo begins at a rather early stage to feed on 

 the yolk outside its own cells, and its relation to the nutritive supply 

 becomes similar to that of the animal developing from a holoblastic 



