SEGMENTATION 131 



mesoderm and that the reduplications in other parts 

 are secondary. 



It is evident that as a physiological process segmen- 

 tation, as well as many other repetitive developmental 

 processes, must involve a rhythmical repetition in 

 physiological conditions of some sort. Ritter and 

 Johnson (Ritter, 1909, 1919 [II, 95-107]; Johnson, 

 1910; Ritter and Johnson, 191 1) have called attention 

 to the significance of these repetitive processes as evi- 

 dence of law and order in organisms and to their bearing 

 on the problem of integration. I have shown that 

 various processes of agamic reproduction in both plants 

 and animals result from physiological isolation (Child, 

 1910a, 191 1 d, 1915c, chap, v; Child and Bellamy, 1919) 

 and have suggested that the physiological conditions 

 determining the reduplications of parts, or meristic 

 series, as Ritter terms them, are essentially similar, 

 except that they concern more or less specialized parts 

 and their repeated development rather than entire 

 organisms. If this suggestion is correct there is little 

 fundamental difference, so far as the physiological con- 

 ditions are concerned, between the two chief theories of 

 segmentation. In the one case the process of develop- 

 ment of a new segment presumably begins in the ecto- 

 derm, since this is primarily the most active layer, and 

 later involves the other layers; in the other case the 

 process begins in the mesoderm and secondarily involves 

 other layers. 



The theories of segmentation have been primarily 

 morphological rather than physiological. When we con- 

 sider segmentation as a physiological process, as it of 

 course must be in every case, it is difficult to mid an 



