Fertebroid Homologies of the Cranium in Vertebralia. 63 



" The dorsal laminae first coalesce in the middle cephalic 

 region, extending forwards and backwards. The cephalic 

 canal is separated into three distinct dilatations or cerebral 

 vesicles, of which the anterior is the largest." Subsequent 

 flexure of the vesicles divides the cavity of the cranium into 

 pro-cranium and meso-cranium, at angles to each other; as 

 development proceeds the meta-cranium and para-cranium 

 cover the posterior lobes of the brain and cerebellum. The 

 neur-arc forming the chondroid tunnel of the cerebrum and 

 cerebellum, from the posterior occipital through the wormian 

 and masto-parietal, coalesce with the ethno-frontal, and 

 cover the ali-sphenoid and spheno-orbital. The basi-cra- 

 nium, composed of, (1.) basi-occipital ; (2.) basi-otic, or 

 basilar ; (3.) basi-sphenoid, or basi-olivare ; from the rostrum, 

 sjihenoidis the chondroid plate of the ethmoid and nasal is 

 prolonged. This chondroid condition is well seen in the 

 cranium of the Salmonidse, where the osseous frontal and 

 parietal are easily forced off the cartilage, and the long 

 basi-sphenoid is equally removable from the chondroid basi- 

 cranium, which extends into the incisor or pre-mandibular 

 palate, sometimes misnamed the vomer. Agassiz main- 

 tained that the centro-chord did not extend beyond the 

 sella turcica ; and Huxley seems to adopt the same opinion, 

 though, strangely enough, he admits that in the Lancelet it 

 extends to the utmost length of the animal, beyond the termi- 

 nation of the myel-encephalon. It may be as well to re- 

 mind the student that the relation of neuraxis and the 

 hsemaxis t6 the digestive or intestinal tube is completely 

 different in the articulate and vertebrate classes. In the 

 segment of the crustacean and entomoid classes, the neur- 

 axis is contained in a small canal below formed by the 

 neur-arcs, but without any kaulon or central stem, and lies 

 beneath the digestive or intestinal canal. The anterior or 

 oral termination, passing through esophageal nervous ring, 

 being the type of the par-encephalon behind with the audi- 

 tory apparatus, while the mesencephalon and prosencephalon, 

 with the optic and olfactory sense capsules, lie before. Thus 

 the pharynx and mouth of the invertebral pass where the 

 pituitary body and infundibulum (possibly the typical rem- 



