214 J. B. JOHNSTON, 
Of these the nucleus medius occupies the same relative position as 
that to which I have given the name nucleus praetectalis in Aci- 
penser. The transverse band of fibres which I have called the epiphysial 
decussation (page 108) has been mentioned by only one author. Hout 
(91) in his paper on the development of the Teleost brain describes a 
transverse band of fibres continuing forward from the posterior com- 
missure (or tectum) to the dorsal wall of the pineal body, whose 
fibres come from the thalamus and part of which enter the pineal 
stalk. This he calls the “Labium invaginatum of the optic ventricle”. 
In position relative to the pineal stalk, superior commissure, posterior 
commissure, and tectum this corresponds perfectly to my epiphysial 
decussation. The commissure of the epiphysis in Mammals is pos- 
sibly the same. 
The epithelial cells in the wall of the epiphysial sac which give 
rise to neurites and among which are found free endings of fibres 
running in the epiphysial decussation have not been described be- 
fore. I have no definite idea as to the probable function of these 
cells. The epithelium forms a membrane roughly similar to that of 
the saccus vasculosus, but there is no fundamental similarity between 
the two. In its minute structure it is to be compared rather with 
the corpus mammillare, in the caudal wall of which the cells present 
almost as simple an epithelial form as do these. I believe that these 
cells are to be regarded as forming a nucleus of the typical character, 
receiving the end-branches of nerve fibres and giving rise to other 
fibres which transmit secondary impulses. Whether the epithelia] 
character of the nucleus indicates that it is in a primitive condition 
or that it is degenerating is an open question. If VAN GEHUCHTENS 
(94) theory of the origin of nerve cells by migration of epithelial 
cells from the ventricular surface is true, then this nucleus is prob- 
ably to be regarded as a poorly developed nucleus which may be- 
come larger and more important in higher forms. The theory of 
VAN GEHUCHTEN is supported by my work on the nucleus taeniae 
and the corpus mammillare (cf. page 141, 118). 
2. Central Grey of Thalamus. 
The central grey in Acipenser shows very slight differentiation 
into special nuclei. I have given to the whole central grey the name 
nucleus diffusus, which has been applied by EDINGER to the un- 
differentiated part of the central grey in Reptiles. I have distinguished 
cells giving rise to the fasciculus longitudinalis posterior and others 
