Literary Notices. xxix 



the author finds in the loss of the more cephalic myotomes through 

 progressive reduction of the gills and hence of the epibranchial and 

 hypobranchial somatic musculature. The lower selachians may per- 

 haps be regarded as marking more nearly than any other existing 

 types that stage of the phylogenesis at which the tendency to extend 

 the gills caudad was finally mastered by the .thereafter dominant ten- 

 dency in the opposite direction, i. e. reduction of the gills, the assim- 

 ilation of spinal metameres into the head and the loss of the first 

 members of the spinal series. Beginning with the selachians, then, 

 we distinguish, using Gegenbaur's terms, the neocranium from the 

 paleocranium ; at the caudal limit of the latter stand the X and XI 

 neuromeres. In the primitive selachians and amphibians certain of 

 the spinal nerves have become fully incorporated into the head (the 

 occipital nerves of Fiirbringer) and these neocranial segments com- 

 prise the protometameres of Sagemehl. In all other vertebrates the 

 process of further assimilation is taking place under our eyes and the 

 result is a series of transitional nerves ("occipito-spinal nerves" of 

 Fiirbringer), whose segments comprise the auximetameres of Sage- 

 mehl. But especial stress is laid upon the fact that in spite of these 

 fusions, throughout the Gnathostomata the neocranial elements can 

 always be sharply distinguished from the paleocranial, i. e. the IX4- 

 X+XI nerve complex never contains spinal elements. 



Regarding the criteria for fixing the boundary between the brain 

 and spinal cord and between the head and the trunk, a very radical 

 position is taken. It follows from the above exposition that in the 

 nervous system this limit will lie between the X-f- XI and the occipi- 

 tal nerves and that so far from being a plane surface, it will zigzag be- 

 tween the nuclei of these nerves. Thus in Hexanchus it runs ven- 

 trally far cephalad to include with the spinal cord the nuclei of the 

 occipital nerves, but also farcaudad to include the whole of the XI 

 nucleus with the brain. This is extended also to the longitudinal 

 tracts, so that the pyramids, e. g., are ranked with the head through- 

 out their length. Among the other tissues, the hypoglossus muscula- 

 ture is relegated to the trunk, while the head includes the ramus lat- 

 eralis vagi, the ramus recurrens trigemini [facialis], the visceral ramus 

 of the vagus and the organs which these nerves supply. While this pro- 

 cedure has more to commend it than the arbitrary limits of His and 

 Huxley, and while some of the most extreme examples of the term- 

 ination of nerves in metameres apparently far removed from the one 

 in which they originate (e. g., the ramus lateralis vagi and the hypo- 

 glossus) can be explained by the cenogenetic migration of the organs 



