232 Journal of Comparative Neurology and Psychology. 



paratus may be expressed as a shifting backward with relation 

 to the vertebrae and muscles, and when the consequent shift- 

 ing of the heart and blood vessels and other organs (see Hoff- 

 mann, 60) is taken into account it is an open question whether 

 the shifting backward has not been greater than the shifting for- 

 ward in the vertebrate body. Shifting should be referred to 

 organs which are known to have some cause for movement with 

 reference to all the others, as the heart is pushed backward by 

 the branchial apparatus, and the trunk myotomes are enabled to 

 push forward by the reduction of those in the head. The 

 analysis of the nervous system and the description of the com- 

 ponents present in each of the spino-occipital nerves is the best 

 guide to the explanation of this region. Now that we know 

 the comparative anatomy, much more is to be gained by the 

 study of the actual shifting of organs in the ontogeny than by 

 further speculation. 



It is probable that all attempts to divide the vertebrate 

 body transversely into primitive regions will have to be given 

 up. The w'hole trend of reasoning in the present paper has 

 been against the idea of palingenetic and coenogenetic head 

 regions (Gegenbaur-Furbringer). Three main objections to 

 this hypothesis may be stated: (i) in Petromyzon the develop- 

 ment of the mesoderm clearly indicates the existence in ancestral 

 vertebrates of a continuous series of similar muscle segments 

 throughout the head and trunk. There is no limit to the 

 palingenetic head to be found in the mesoderm. This seems to 

 be the best key to the interpretation of the conditions in selach- 

 ians and higher vertebrates as well. 



(2) It is by no means certain that the pharynx with its 

 gill slits was confined in primitive vertebrates to the palingenetic 

 head. There are indications that the gills once extended far 

 into the trunk and that the specialization of the branchial nerves 

 had its beginning from that condition. 



(3) The interpretation of the nervous system from the 

 functional point of view strongly favors the view that the ances- 

 tral vertebrate had a very slightly specialized head region. It 

 is certain that the fundamental divisions of the nervous system 



