Herrick, Morphology of Brain of Bony Fishes. 65 



the cluster of cells on the dorsal aspect of the hypoaria. In the 

 eel {Anguilla) the nidulus ruber is relatively very large and does 

 not have the obvious glomerular structure of the drum. 



In Hyodon the nidulus is very simple and does not assume 

 the glomerulary phase. Among ganoids the nidulus is diffuse, 

 but has the same structure at least in Lcpidosteus. 



In the young bull pout {A/niurus), the nidulus ruber is one 

 of a group of similar masses and, while there is very much less of 

 the suffusion characteristic of the nidulus than in adults, the 

 grouping of cells into rosette-like clusters, the presence of a 

 great number of blood vessels and the interblending of the 

 multipolar switch cells is characteristic. In a somewhat later 

 stage the nidulus becomes more compact and enclosed in a loose 

 capsule of fibres, its cells acquire a radial arrangement, and the 

 gelatinous suft''usion becomes marked. The course of the com- 

 missura horizontalis through the nidulus, then cephalo-dorsad to 

 a point adjacent to the ventricle, immediately ventrad of the 

 posterior commissure, is easily traced. The ventral pyramidal 

 bundle lies laterad of it, and is also distinct. 



In specimens of young Belone belone ( Scomberesocidae ), 

 measuring between three and four centimeters, the " nucleus 

 rotundus " is already well developed and consists of a nearly 

 spherical body, lying dorsad of the hypoaria. The walls of 

 the sphere are formed by thickly set radial cells of a fusiform 

 type. The apex prolongations extend inward, and at the same 

 time cephalad. The sphere is thus enclosed on all sides except 

 cephalo dorsad, where is a gap through which a strong bundle of 

 fibres emerges. Outside the cellular envelop is a less dense 

 zone, outside of which are numerous fibres of the pyramidal 

 and other systems. Within are numerous blood vessels and a 

 gelatinous mass which also contains some blanched cells, like 

 those seen in adults. There are also a number of the " switch- 

 cells," which connect different fibres. Mesad of this body is a 

 bundle of nerve fibres, which is probably a part of the dorsal 

 peduncular bundle from the cerebrum. From the comparatively 

 sparing embryological material at hand we consider it probable 

 that the whole cell cluster springs from the median part of the 

 optic ventricle (or third), being derived from one of the many 

 clusters of neuroblasts which arise there. 



