42 2 SENESCENCE AND REJUVENESCENCE 



rate begins to decrease, the rate in more posterior regions is in- 

 creasing more rapidly than in the head, and the result is a partial 

 physiological isolation of the posterior region and the formation 

 of a new segment. Similarly, physiological isolation of the pos- 

 terior region from the first segment results in the formation of the 

 second, and isolation from the second in the formation of the 

 third. But by this time the rate of metabolism in the head-region 

 is decreasing, and a little later it begins to decrease in the first, 

 then in the second and the third segments. Sooner or later this 

 process leads to partial physiological isolation of the posterior end 

 and, if food is present to provide energy and substance for growth, 

 another segment is added posteriorly, and so on. 



In the Crustacea the process is essentially similar. In the lower 

 Crustacea the earliest larval stage represents, as in Nereis, the head, 

 and three segments with their appendages, and new segments are 

 added successively at the posterior end. Fig. 198 shows a stage in 

 the metamorphosis of the fairy shrimp Branchipus. The original 

 larval form in this case consisted of the head and the first three 

 segments to which the three pairs of large appendages are attached 

 in the figure, and to this new segments are successively added at 

 the posterior end. The figure shows a stage in which a large 

 number of segments have already formed, but are not yet fully 

 developed. 



In the insects and vertebrates the formation of the segments 

 occurs before hatching, but is in all probability a similar process. 

 The changes called metamorphosis in the insects belong to a much 

 later stage of development. Here the larval form, which has fed 

 and grown for a time and has acquired a large nutritive reserve, 

 undergoes transformation into the mature form, the imago, during 

 the pupal stage which usually shows little or no movement and does 

 not feed. In this case the changes seem to be the result of aging 

 of certain of the larval organs in consequence of which growth and 

 development of certain parts previously inhibited now becomes 

 possible. In some insects many of the larval organs actually die 

 and undergo complete resorption or degeneration. In some other 

 invertebrates parts of the larva die and are cast off bodily when 

 metamorphosis begins. 



