184 Journal of Comparative Neurology and Psychology. 



a given segment depends upon what other organs are present 

 in that segment. This very obstacle suggests the way in which 

 the nervous system may be used with special profit, namely, 

 as a key to the interpretation of the facts secured by the study 

 of the other organs. From the point of view given by the 

 theory of functional divisions we get an advantage in the study 

 of head segmentation which may be stated as follows. The 

 scheme of functional divisions serves to define a complete or 

 typical segment and so enables us to determine whether all the 

 proper parts are present in a given segment. A complete seg- 

 ment of head or trunk of a typical vertebrate should consist of : 

 (a) derivatives from the ectoderm (skin), from the entoderm 

 (gut), from the dorsal mesoderm (somatic musculature), and 

 from the lateral mesoderm (visceral musculature), and (b) the 

 corresponding four main divisions of the ner\'ous system : so- 

 matic sensory, splanchnic sensory, somatic motor, splanchnic 

 motor. With this clearly in mind we shall be able to say when 

 a segment is complete, when incomplete, what constituent has 

 disappeared, and for what probable reason. When we can say 

 this for each segment of the head the problem of segmentation 

 will be solved. 



In order to proceed by this method it will be necessary 

 first to give a summary of what has been established by previ- 

 ous workers on the various systems of organs. It is not the 

 intention to give a critical review of the literature of head seg- 

 mentation or to undertake the discussion of disputed points ; 

 but only to give a brief statement of the facts that may be used 

 in attempting to reconstruct the primitive vertebrate head. It 

 is believed that a constructive effort aiming to correlate all the 

 well established facts from a new point of view is more valu- 

 able than criticism or the weighing of one set of facts over 

 against another. 



2. Number and relations of mesoilennic somites. 



Since the work of later authors has given strong confirma- 

 tion to the main results of van Wijhe (119) we may use his 

 nomenclature in referring to the somites. 



