4i8 SENESCENCE AND REJUVENESCENCE 



In the frog and salamander, the only other vertebrates for which 

 embryonic susceptibilities have been determined, the changes are 

 very similar to those described for other forms. From the time of 

 fertilization on, through cleavage, gastrulation, and the formation 

 of the embryo, and somewhat beyond the stage of hatching, the 

 average susceptibility increases. As in the fishes, the results in the 

 later stages are perhaps complicated by the increased metabolic 

 activity connected with the functional activity of special organs 

 and with movement, or by changes in the nervous system, but as 

 regards the earlier stages this is certainly not the case. 



All of these data, as well as those on oxygen consumption, are 

 in full agreement with the observed facts of development. It is 

 well known that as cleavage goes on the rate of cell division is 

 accelerated and other developmental changes proceed more and 

 more rapidly up to a certain stage. In general the rejuvenes- 

 cence of certain parts of the embryo, and particularly of the apical 

 region, where the metabolic rate is originally highest, proceeds more 

 rapidly than that of other parts and is completed earlier. 



THE MORPHOLOGICAL CHANGES DURING EARLY DEVELOPMENT 



The morphological changes during the period of increasing 

 susceptibility consist in an increase of nuclear as compared with 

 cytoplasmic substance and in the decrease and disappearance of 

 the yolk in the cytoplasm and the increase of the amorphous, 

 undifferentiated, or embryonic cytoplasm; often also, particularly 

 in the later stages, the new morphological features connected with 

 the new process of differentiation begin to appear. The increase, 

 both absolute and relative, -in total nuclear volume is a character- 

 istic feature of embryonic development in animals and is evident 

 from observation. It has often been stated that the nuclear 

 volume or nuclear substance increases in geometrical progression 

 during this period, but measurements, so far as they have been 

 made, indicate that this is by no means always the case. Godlewski 

 ('08) has found that in the sea-urchin from the four-cell to the 

 sixty-four-cell stage the nuclear volume does increase almost in 

 geometrical progression, while from the sixty-four-cell stage on 

 there is but little further increase. During the period of nuclear 



