26 Journal of Comparative Neurology. 



with the ventral surface of the medulla and surrounding the 

 roots of the XII nerve. 



B. The Mid Brain. 



a. Tectum opticum (Figs. 2, 12, 13, 14. 30). 



As already noted, the caudal part of the tectum presents 

 the characteristic appearance of the bi-lobed fish tectum, the 

 two sides being connected by the dorsal decussation. In its 

 cephalic part the two lobes become separated, there is no dorsal 

 decussation, and the roof is a choroid plexus. The plexus grows 

 wider cephalad as the lateral walls recede and the tectum disap- 

 pears at the level of the posterior commissure. 



The tectum presents a well defined grey zone adjoining 

 the cavity made up of from three to six closely packed layers 

 of cells, and a fiber zone which makes up more than three-fourths 

 of the thickness of the wall. The fiber zone contains a large 

 number of cells scattered irregularly through it. Upon the 

 outer surface of the tectum throughout its whole extent there 

 are isolated cells at short intervals which are pyramidal or bal- 

 loon shaped, with the large end against the limitans externa 

 and the smaller end penetrating the fiber zone. The tectum has 

 a much larger number of cells in proportion to its whole vol- 

 ume or the volume of its fiber zone than there are in Acipenser. 



The cells of the tectum are not so well differentiated as in 

 Acipenser, but some of the same types are to be recognized. 

 The cell and fiber zones are not distinguished in Golgi prep- 

 arations and for the study of the minute structure the cells may 

 be divided into two groups, deep and superficial cells. The 

 deep cells are vertical or horizontal. The vertical cells have 

 ovoid or spindle shaped cell bodies which are usually within 

 the cell zone and sometimes have central processes reaching the 

 cavity. Their dendrites are few in number and rise toward the 

 periphery. These resemble the cells denominated " type A " 

 in Acipenser but are not so well differentiated. The neurites are 

 not well impregnated and usually appear as only short stumps 

 starting toward the surface, but in a single case a well marked 

 short neurite was found well impregnated. The dendrites branch 



