Turner, Morphology of the Avian Brain. I3i 



sub-spheroidal to a flattened sub-ellipsoidal body. When 

 of the latter shape, its major axis is parallel to the meson. 

 In hivmatoxylin and in aluminium-sulphate cochineal prepa- 

 rations, this nidulus consists of a dense, deeply stained outer 

 shell, within which is a solid core of lighter material. 

 Exactly what the cell structure is, I have been unable to 

 determine. This nidulus is surrounded by fibre tracts, and 

 one originates in it. In all probability this body is the 

 homologue of the testis of the mammalian corpora quadri- 

 gemina. 



Nidulus iftfcriiis (Plate XV, Fig. 6). — Near the ventral 

 surface of the mesencephalon, mesad to the external optic 

 tracts and adjoining the tuber cinerium, there is an elongated 

 lenticular nidulus. This nidulus extends from the junction 

 of the mesencephalon with the metencephalon cephalo-mesad 

 about half way to the optic chiasm. It is a dense nidulus, 

 containing scattered bipolar and multipolar nerve cells. 

 These cells are large and irregular. In hasmatoxylin and in 

 aluminium-sulphate cochineal preparations, these cells are 

 densely stained, and each presents an elongated dense 

 nucleus. This nidulus is associated with both the pedun- 

 cular fibres and with the fibres passing to the decussatio- 

 inferior. Bellonci(') has called this body the " peduncular 

 nidulus." But, since this name has previously been applied 

 by Stieda to the nidulus which lies immediately laterad to 

 the tract of the third nerve, and since two niduli of the same 

 name in the same brain can but breed confusion, the appella- 

 tion " nidulus inferius" is proposed. 



Hahena. — cephalad to the epencephalon, there is a slight 

 dome-shaped protuberance from the dorsal surface of the 

 diencephalon. This is the habena. In the brains examined 

 this protuberance contains an ill-defined nidulus, the nidulus 

 of the habena. 



Mesencephalic nidulus of I he fflh nerve (Plate XV, 



1 Op. cit., taf. v. Figs. 1-4, x. 



