ii8 Journal of Comi'aka tive Nei'koi.oov. 



the mesencephalic ventricle, there are two prominent niduli. 

 One of these I have called nidulus lenticularis, and the other 

 nidnlus sub-pyriformis. 



Nidulus lenticularis (Plate XV, Figs. 5, 7-10). — This is 

 a large lenticular nidulus, w^hich is situated in the central 

 part of the optic lobe. It lies caudad to the mesencephalic 

 ventricle, and immediately entad to the fasciculus internus. 

 The laterad extremity of this nidulus is about on a level with 

 the laterad extremity of the mesencephalic ventricle. The 

 major axis of this nidulus is oblique to the ventricle, the 

 cephalo-laterad extremity of that axis being much nearer the 

 ventricle than the caudo-mesad extremity. In different bird 

 bi-ains the size of this nidulus varies. In Swainson's thrush 

 {^Hylocichla s-xainsoni) the major axis of this nidulus is from 

 800 to 1,000 micro-millimetres long, while the minor axes are 

 respectively about 300 micro-millimetres and 135 micro- 

 millimetres long. In the blue bird [Sialia sia/is) the major 

 axis is about 916 micro-millimetres long, while the shortest 

 axis is about 190 micro-millimetres long. 



Histology. — The principal cells of this cluster are large 

 and gibbous, and have their longitudinal axes parallel to the 

 longest axis of the nidulus (Plate XVI, Fig. 7). These cells 

 rank among the largest in the brain. In Swainson's thrush 

 [Hylocichla sivainsoni) these cells are from twenty-two to 

 twentysix micro-millimetres long and from eight to sixteen 

 micro-millimetres broad. In the blue bird [Sialia sialis) 

 the same cells are from sixteen to nineteen micro-millimetres 

 long and from six to ten micro-millimetres wide. In shape 

 these cells are pyramids. A prominent fibre projects from 

 the apical process of each cell, while smaller fibres project 

 from each of the several basal processes. When stained with 

 hematoxylin or with aluminium-sulphate cochineal, the cells 

 are densely colored, and each possesses an elongaged or sub- 

 spherical dense nucleus and a denser nucleolus. 



In some of my sections the cells of this nidulus appear to 

 be flask-shaped (Plate XVI, Fig. 11). These cells, however, 



