PROCESS OF REGENERATION 123 



cyanide, disintegration begins at the anterior end, then at the 

 posterior end, and the two waves of disintegration meet at about 

 the middle (fig. 18). As growth proceeds, the susceptibility of 

 the head continually falls, but that of the posterior end remains 

 high because of its unceasing growth which keeps it young. These 

 experiments of Child and myself show that the posterior region 

 of high metabolic rate which is characteristic of the adult annelid 

 arises A-ery early in the embryonic de\'elopment. 



Fig. 17 Stage in the disintegration of a later embryo than figure 16, showing 

 two regions of high susceptibility. 



7. SiaN//t<ir}/ 



These various species of oligochaetes which have been examined 

 form a series, then, with respect to the axial gradient and also, 

 as I shall show later, with regard to regeneration. In Aeolesoma 

 the gradient is of the primary type with some indications of a 

 posterior region of high metabolic rate. In the naids, the pri- 

 mary gradient exists in the zooids only, and is modified in the 

 adult b}' the development of a posterior region of high metabolic 

 rate, which occupies about one-third of the body. In this region 

 the gradient runs in the reverse direction from that of the pri- 

 mary gradient; it may be regarded as a secondary gradient im- 

 posed upon the primaiw gradient as a result of the annelid method 

 of growth. In Lumbriculus, the secondary gradient is more 

 extensive than in the naids, including more than the posterior 

 half of the body ; and in the tubificids it comprises all of the body 

 except the first few segments. 



THE JOURNAL OF EXPERIMENTAL ZOOLO(;Y. VOL. -'0. NO. 2 



