290 



SCIENCE PROGRESS 



which appears to be quite constant throughout the vertebrate 

 series. 



It is now tolerably certain that the two habenular ganglia 

 are primarily related to the parietal or pineal sense-organs 

 (developed from the pineal outgrowths), with which they are 

 still connected in some cases (e.g. lampreys) by nerve-fibres. 

 One of the two pineal outgrowths, forming the so-called " stalk " 



MB 



Fig. I. — Diagrammatic sagittal section through the brain of a typical vertebrate 

 embryo (the pineal organs as in Lacertilia). 



C, cerebellum; CD., dorsal (mantle) commissure; C.H., cerebral hemisphere; C.H.S., 

 superior (habenular) commissure; C.P., posterior commissure; C.V., ventral (anterior) 

 commissure; D.S., dorsal sac; Ep. i, anterior epiphysial outgrowth (pineal eye); Ep. 2, 

 posterior epiphysial outgrowth (pineal stalk) ; F.M., foramen of Monro; Inf., infundibulum ; 

 M.B., mid-brain; Med., medulla; N. 1-12, neuromeres; Op. Ch., optic chiasma; Par., para- 

 physis; Pros., prosencephalon ; Thai., thalamencephalon ; Vel., velum transversum. 



(with or without a terminal enlargement), remains connected 

 with the brain-roof either permanently or at any rate for a 

 long time, at a point which lies between the superior and 

 posterior commissures. From each of the habenular ganglia 



