228 BULLETIN OF THE UNIVERSITY OP WISCONSIN. 



GROSS ANATOMY. 



Prosencephalon. The prosencephalon is composed of two 

 rounded cylindrical bodies which are sharply separated by a 

 longitudinal fissure as far as the anterior closing plate. The 

 cylindrical bodies are about 8.8 mm long and about 2.5 mm in 

 diameter; they are slightly concave along their lateral border 

 and straight along their median side, and at their posterior end 

 they are more diverging than in front. Measured dorso-ven- 

 trally they are thickest at their posterior end and gradually 

 narrow towards the origin of the olfactory nerve. In cross sec- 

 tion their median surfaces are shown to be flattened while their 

 outer are convex; the union of the two surfaces is marked by 

 a sharp angle. As there is no sharp line of demarcation between 

 the olfactory lobes and the hemispheres, for convenience of de- 

 scription they will be considered together ; and thus each hemi- 

 sphere is to be considered as composed of the olfactory lobe and 

 the hemisphere proper. 



On the other hand, in Rana and in the Chelonia the olfactory 

 lobes can be readily distinguished from the cerebral hemispheres. 

 In all these cases, however, the surface of the prosencephalon 

 is perfectly smooth and is without convolutions. 



Concerning the question as to whether the olfactory portion 

 deserves setting apart as a separate segment there is at pres- 

 ent a variance of opinion. 



Steiner, working from a physiological standpoint, argues from 

 experiments carried on in the shark, that the prosencephalon 

 of every vertebrate has developed phylogenetically out of the 

 olfactory organs. 



Tlialamence photon and Mesencephalon. These two are 

 united into a short cylindrical body with no sharp line of de- 

 marcation between them. This body is flattened or even slight- 

 ly concave on the dorsal and ventral surfaces. These surfaces, 

 with the exception of the median groove on the dorsal side, are 

 all unbroken, there being no elevations or other external indi- 

 cations to mark the position of the optic lobes in the mesenceph- 

 alon. In this respect the brain of the Necturus differs greatly 



