1891.] Recent Studies of the Vertebrate Head. 34I 
neural axis anteriorly ; it is intimately connected with the sense 
organs, eye and nose; it gives off at least two pairs of sensory 
nerves with peripheral ganglia; it possesses ganglionic centers of 
coordination ; it has an enlarged central canal with three diverticula, 
two optic and one olfactory ; it is the largest part of the nervous 
system in early stages ; it possesses a cranial flexure ; it shows a 
differentiation into ganglionic and fibrous tracts. The large col- 
lections of ganglion cells just posterior to the thalamoccele are 
homologous with the medullary nuclei of other vertebrates. In 
the ontogeny of other vertebrates the brain passes through a con- 
dition which remains as adult in Amphioxus. All the sense 
organs of the anterior end of the body of Amphioxus are prob- 
ably paired. The eye-spot is the forerunner of the vertebrate 
eye, and shows several stages in development. The pigment of 
the eye-spot is contained in cells that lie normally inside the 
bounds of the nerve-mass. The pigment bodies form a part of 
segmental sensory structures. Each of the pigment bodies 
forms a deposit in an ameeboid cell. The pigment of the 
the axial nervous system of Amphioxus is in process of migra- 
tion towards the anterior end of the body. The vertebrate ear 
has developed within the phylum above Amphioxus, and arose 
from one of the primary sense organs of the lateral line system, 
at a period phylogenetically later than the formation of the canal 
system of these sense organs. The ear capsula does not separate 
two morphologically different p®rtions of the brain. The higher 
sense organs of all the Cyclostomata are all paired. The 
parietal-pineal eye of the Cyclostomata and other vertebrates has 
been developed from a median portion of the pigmented eye of 
Amphioxus. The neural axis of all vertebrates is coéxtensive 
with that of the chorda. The pituitary prominence of the skull 
of vertebrates does not mark a fixed point. The chondro- or 
ossicranium possesses no more segmental value than the intestine. | 
The head-cavities possess relatively the greatest importance before 
a primordial cranium has made its appearance. The hypophysis 
arose in the vertebrate phylum long after the appearance of the 
chorda, and was connected with the infundibulum. It arose as a 
‘See Rabl. Theorie des Mesoderms. 
