204 Journal of Comparative Neurology and Psychology. 



stomial area is extremely short in the early embryo and in fact 

 assumes appreciable size and definite form only after the period 

 at which the relations between the brain plate, ectoderm and 

 entoderm above dealt with are in evidence. The term "anterior 

 end of the head" as used here really applies to the base of the 

 prostomium, or the most anterior point to which the two prim- 

 ary germ layers extend. The anterior tip of the prostomium 

 consists of ectoderm alone and could be located only by reference 

 to the point determined upon as the result of the above examin- 

 ation. 



7. TJie segment containing N . III. Segment^. 



Neuromere v, somite i, N. profundus, N. III. 



To fully describe a given segment it is necessary to de- 

 termine what representatives are present of the skin, dorsal 

 mesoderm, lateral mesoderm, and gut. and of the somatic sensory, 

 somatic motor, splanchnic motor and splanchanic sensory divi- 

 sions of the nervous system severally related to these structures. 

 If any of these components of a true segment are absent their ab- 

 sence should be accounted for, and a functional relation between 

 the nerves recognized and the segmental organs present should 

 be clearly shown. 



In determining the neuromere to which a somatic motor 

 nerve belongs it should be remembered that these nerves 

 emerge from the brain as ventral roots in the constrictions be- 

 tween the neuromeres. Since a neuromere is considered to be 

 the convex segment between two constrictions, it is most con- 

 venient to assign the ventral roots to either the preceding or 

 the following neuromere. In the trunk region where in all 

 higher vertebrates the dorsal and ventral roots unite in the 

 spinal ganglia the ventral root should be assigned to the neuro- 

 mere with which the dorsal root is connected. The relative 

 position of the dorsal and ventral roots, the neuromeres, and 

 the myotomes will be discussed in a later section (Sec. 19)- 

 here the result of that discussion will be anticipated and the 

 ventral roots assigned to neuromeres next caudal to the place 

 of origin of the roots. 



