34 C. M. CHILD 



ism in the behavior of the two separate mesoblast bands on the 

 two sides of the body indicates the existence of some physio- 

 logical coordinating factor external to the mesoblast bands 

 themselves and we can expect to find such a factor only in the 

 ectoderm. 



In Planaria the posterior zooids are distinguishable physio- 

 logically in various ways, for example, by the independent motor 

 reactions of the zooid under certain conditions, by the suscepti- 

 bility gradient in the ectoderm as well as in other parts, and by 

 the capacity for head-formation in isolated pieces (Child, '11), 

 but no certain morphological indications of the existence of 

 these zooids have ever been discovered. The susceptibility 

 method in its present form is not sufficiently delicate to dis- 

 tinguish the earliest stages in segment-formation, but the data 

 presented above on developmental modification in annelids 

 show that segment-formation may be more or less completely 

 inhibited, and we kilow that the ectoderm is much more suscep- 

 tible than the mesoderm to the concentrations of agents use 

 for this purpose. This fact is of course only suggestive, not 

 conclusive. 



These various lines of evidence seem to me to favor the con- 

 clusion that the formation of a segment is physiologically a new 

 individuation resulting from physiological isolation and funda- 

 ment all}' similar to embryonic development in its earlier stages. 

 According to this view, the head region of the annelid represents 

 the primary individual, the primar\' gradient and the addition 

 of segments is the result of successive physiological isolations 

 resulting from lower specialization and greater growth capacity 

 of the dorso-posterior region of this individual and bringing about 

 a series of repro'ductions producing individuals, which are pre- 

 vented from developing a head by the process of integration 

 into a 'composite' individual through the development of the 

 nervous system and the extension of nervous control which 

 proceeds from the apical or anterior end. 



The process of segmentation is then fundamentally similar 

 to the process of zooid-formation in Planaria and other forms 

 (Child, '11: '15 b, Chapter V), but the segments of the annelid 



