Johnston, Morphology of the Head. 261 



ton and somatic muscles and in the relation of the dorsal and 

 ventral nerve roots to one another and to the myotomes. The 

 formation of a rigid cranium has had very important consequnces 

 in the disappearance of postauditory myotomes and the develop- 

 ment of complicated conditions in the occipital region. To this 

 also is due the fact that certain myotomes have been released 

 from their attachment to a segmented skeleton and have come 

 to move the eyeball. It may be said that the same cause has 

 indirectly determin-ed the relation of the nerve roots to the 

 myotomes. In cyclostomes, where the buccal apparatus has 

 demanded the preservation of the dorso-lateral portion of the 

 postauditory myotomes, the nerve roots are covered by these 

 very much as in the trunk. In gnathostomes, however, when 

 postauditory myotomes completely disappear the nerves are given 

 free access to the ectoderm. When trunk myotomes shift for- 

 ward it is their ventro-mesial portions which become attached to 

 the cranium and so the nerves retain their superficial position. 

 The union of the dorsal and ventral roots in the trunk is a late 

 development and the head has retained the more primitive con- 

 dition. In the constitution of the dorsal and ventral roots also 

 the head has retained the primitive condition, while the 

 trunk nerves have been somewhat modified at least in 

 higher vertebrates. The dorsal roots originally contained 

 the somatic sensory, splanchnic sensory and splanchnic 

 motor components, as in Araphioxus. In the trunk a part of 

 the splanchnic motor component has come to run in the ventral 

 root and perhaps other modifications have taken place. 



SUMMARY OF THE CHIEF CONTENTS. 



1. The primitive vertebrate was a segmented animal with 

 probably very slight cephalization. 



2. Each segment consisted of derivatives of the ectoderm, 

 dorsal mesoderm, lateral mesoderm and entoderm, and of 

 somatic sensory, somatic motor, splanchnic motor, and splanch- 

 nic sensory divisions of the nervous system primarily related to 

 the skin, the myotomes, the visceral muscles, and the visceral 

 surfaces respectively. The definite relations existing between the 



