DEVELOPMENT OF THE NERVOUS SYSTEM. 69 



clefts has been gradually reduced from behind forwards (except 

 in Myxinoids), probably because the greater development of a 

 few anterior gills and the concentration of organs thus made 

 possible is more advantageous. 



At the same time that the disappearance of the more caudal gills 

 reduces the number of aortic arches and tends to bring the heart 

 farther forward, the expansion of the permanent gills actually 

 pushes the heart caudally farther into the trunk. Thus in selachians 

 it is shown that the opening of the duct of Cuvier into the sinus 

 venosus moves back during development from a position in the 

 second trunk segment to a position in the tenth trunk segment 

 (Hoffmann). 



We have to think of the development of the head as a gradual 

 process which has been taking place during the phylogenetic 

 development of vertebrate animals, the causes for which are to be 

 found in the conditions of life, the food, habits, modes of locomo- 

 tion, importance of the various senses, etc. The ancestors of 

 vertebrates were very slightly specialized at the anterior end and 

 the various specialized structures formed there make up the head. 

 Special sense organs are formed in the head region (acustico- 

 lateral system, taste buds, ear, eye, olfactory organ) and each of 

 these profoundly affects in a characteristic way the structure of 

 the brain, and often modifies surrounding organs, determines 

 the form of the skull, etc. The formation of the rigid cranium 

 has come about largely for the protection of the sense organs 

 and the existence of this cranium has led to the disappear- 

 ance of the postauditory somites and to the modification of the 

 preauditory myotomes into muscles which move the eye-ball. 

 Some of the most striking marks of the specialized head are due 

 to the disappearance of various structures, the postauditory 

 somites, the ancient mouth, certain gills, etc. The disappear- 

 ance of these organs has led to the disappearance of their nerves 

 and to great changes in the central nervous system. Furthermore, 

 both the rise of new structures and the passing away of old ones 

 have brought about shifting of organs which is of great importance 

 in determining the structure of the nervous system. Of chief 

 interest is the shifting of nerve roots due to these or other causes. 



