Johnston, The Radix Mesencephalica Trigeniini. 609 



In the liiiman embryo of 15,5 mm. the cells in question are large 

 and possess large clear nnclei so that they are strikingly different 

 from the surrounding cells. This is shown in Fig. 8, which also 

 serves to compare these cells with the cells of the motor nucleus of 

 the trigeminus in the same embryo. 



3. The Embryonic Origin of the Cells. — 1 have made no effort to 

 trace the whole history of these cells because I have no series of 

 embryos of any form in which -the cells are numerous. However, in 

 all stages that I have examined the cells in the tectum have the same 

 position as in the adult. As they lie in or adjacent to the roof plate 

 of the neural tube it is clear that they have their origin in this region. 

 It seems most probable that they have been derived from the neural 

 crest during development, having been enclosed in the neural tube as 

 are the giant ganglion cells of the spinal cord. There is a close 

 resemblance between the cells in the spinal cord and those in the 

 tectum in young fishes and adults. In mammalian embryos, those 

 cells which come to occupy the locus coeruleus in the adult lie not at 

 the dorsal border of the medulla but near the sulcus limitans (Figs. 

 13 and 14). At the level of the velum and IVth nerve the cells are 

 near the dorsal border- of the brain throughout life. 



4. The Course of the Badix Mesencephalica. — In all the forms 

 studied the general course of the bundle of large fibers is the same. 

 Running caudally through the lateral part of the tectum the bundle 

 passes through the lateral part of the velum medullary anterius or 

 corresponding region, continues on the internal surface of the 

 l)racliium conjunctivum and internal face of the lateral wall of the 

 fourth ventricle (locus coeruleus), passes over the motor nucleus of 

 the trigeminus, turns laterad at the same time and crosses the motor 

 root bundles at a greater or less angle, and joins the sensory root at 

 the y)oint of exit or joins the spinal trigeminal tract some distance 

 caudal to the root. As it passes through the velum meduUare the 

 l)undle, in the forms studied by me, always holds the same relation to 

 the IVth nerve and other bundles in the velum: namely, it runs 

 lateral and dorsal to the ascending root portion of the IVth nerve 

 and ventral to the trunk of this nerve after its decussation. The 

 radix mesencephalica trigemini also runs beneath the decussatio veli 



