ANATOMY OF A CYCLOSTOME BRAIN 643 



under the cerebellum and quite absent superficially in the pos- 

 terior part of the mesencephalon. It reappears as a very slight 

 groove in the anterior part of the mid-brain and posterior part 

 of the thalamus. It is interrupted in the mid-thalamic region 

 by the lobus ventralis thalami, and may be represented farther 

 forward in the sulcus medius and a wide shallow depression ex- 

 tending from the later into the preoptic recess. 



This sulcus separates a ventro-lateral motor lamina from a 

 dorso-lateral sensory lamina of the neural tube, and throughout 

 the medulla oblongata the type of cellular differentiation of these 

 two laminae is very characteristic. In the mid-brain these differ- 

 ences, though less conspicuous, are still evident. The ventral 

 lamina is characterized by the giant cells of Miiller, of which 

 five pairs fall within the limits of our model (fig. 3) . Even when 

 these cells are left out of account, the motor lamina exhibits 

 larger and more irregularly arranged neurones than the sensory. 

 Throughout the mid-brain, even where the limiting sulcus is not 

 evident on the ventricular surface (fig. 12), this difference in the 

 character of the neurones, reinforced by a thickening of the epen- 

 dyma in the site of the sulcus and by other internal peculiarities, 

 enables us to restore the missing parts of the sulcus with a high 

 degree of probability, as indicated by the row of crosses in figure 

 3. But in the thalamus under the lobus medius thalami all 

 criteria fail, the locus of the sulcus limitans being here obliterated 

 internally, as well as externally (fig. 9). 



The paired giant cells of Miiller have been so fully described 

 and pictured by various authors that nothing is necessary here 

 beyond indicating on the diagram (fig. 3) their size and position. 

 In addition to the five pairs of huge multipolar cells, there are 

 in the motor column a considerable number of pale round cells 

 much smaller than the giant cells, but still conspicuously larger 

 than the ordinary cells of the motor column. They are commonly 

 situated in the lateral border of the cell column and are usually 

 unsymmetrically arranged. 



Nervus oculomotorius. Three nuclei of the III nerve have been 

 described in cyclostomes. Tretjakoff ('09, p. 678) in Ammocoetes 

 mentions dorsal, ventral and lateral nuclei. The lateral nucleus 



