624 'Journal of Comparative Neurology and Psychology. 



point at which all the confusion of the two kinds of fibers arises, this 

 section is drawn in Fig. 20. The motor roots are cut obliquely, the 

 sensory root is cut nearer to its point of exit. The crossed motor 

 bundle crosses the mesencephalic bundle at such an angle that the 

 two kinds of fibers can be clearly distingTiished. It is not necessary 

 to describe other sections of this series. Cephalad from this the 

 mesencephalic bundle has its usual position. The relative position 

 of the cell column of the mesencephalic root and the motor nucleus is 

 clear in these sections. The motor nucleus extends forward six 

 sections from the one drawn in Fig. 20, and the cells of the mesen- 

 cephalic root appear in the fourth section cephalad from that of Fig. 

 20, so that there is an overlapping of the two for three sections of 

 this series or six sections of the complete series. The cells of the 

 mesencephalic root lie one millimeter or more dorsal to the motor 

 nucleus and are separated from it by the bulk of the mesencephalic 

 bundle itself and by the crossed motor bundle. 



In this embryo the mesencephalic root, coming from its cell column 

 which lies dorsal and cephalic to and .separated from the motor 

 nucleus, nms ventro-laterad lateral and caudal to the motor bundles, 

 which it crosses obliquely, and leaves the brain in the heart of the 

 sensory root. 



Foetus of 42 cm., transverse sections. The 42 cm. foetus is taken 

 up next in order to compare the transverse sections with those just 

 described. One section from each half of the brain is shown in Figs. 

 21 and 22. The two sections are nearly at the same level. In the 

 section through the left half of the brain (Fig. 21) the cerebellar 

 portion was broken away as indicated by the dotted line ; in the right 

 hand section (Fig. 22) the reticular formation was broken as indi- 

 cated by the dotted line, but neither of these breaks in any way 

 affected the structures under consideration. An examination of the 

 figures will show that the chief relations here are identical Avith those 

 in the earlier foetus. Medullation is now much more general. The 

 reticular formation contains many medullated fibers and the pons, 

 acustic area and cerebellum show fairly numerous fibers. The 

 sensory root is cut just at the point where a part of its fibers are 

 turning into the spinal trigeminal tract, which is better formed on 



