MYELENCEPHALON. 425 



outer layer, the marginal velum, is composed of neuroglia; the 

 middle, or mantle layer, of neuroblasts; and the inner, or ependy- 

 mal layer is made up of columnar epithelial cells. 



The cells of the inner layer become ciliated and form the lining 

 of the ventricle. 



The mantle, or middle layer undergoes most development. 

 Its neuroblasts form the substantia reticularis and the cerebral 

 nerve nuclei and other nuclei of the medulla oblongata. In the 

 dorsal zone the neuroblasts form the terminal nuclei for the eighth, 

 ninth and tenth cerebral nerves and the nucleus funiculi gracilis, 

 nucleus funiculi cuneati and nucleus of the spinal tract of the tri- 

 geminal nerve. As early as the fourth week axones may be traced 

 from the nucleus funiculi gracilis and nucleus funiculi cuneati, 

 ventro-medially, toward the point where they very soon form the 

 fillet decussation. Neuroblasts which have wandered from the dor- 

 sal zone form the arcuate, the olivary and accessory olivary nuclei. 

 The olivary nuclei are developed quite late in the intrauterine 

 life (sixth month). The nucleus of the cuneate funiculus, the 

 nucleus of the spinal tract of the n. trigeminus, the olivary and 

 arcuate nuclei are all products of the rhombic lip (Cunningham). 

 From the neuroblasts of the ventral zone are developed the gray 

 matter and fibers of the substantia reticularis alba et grisea, and 

 the motor nuclei of the twelfth, eleventh (cerebral part), tenth 

 and ninth pairs of cerebral nerves. 



The neuroglia layer, or marginal velum, forms the support- 

 ing matrix for the tracts of fibers in the medulla. By the third 

 month the funiculus gracilis and funiculus cuneatus, extensions 

 of the same fasciculi in the cord, have grown up to their terminal 

 nuclei in the medulla. The restiform body is at that time well 

 developed, and the tracts of the lateral area of the medulla are 

 visible. The medial longitudinal bundles appear near the median 

 raphe in the ventral zone at about the same time; and, ventral to 

 them, fibers from the fillet decussation insinuate themselves and 

 form the interolivary stratum of the medial fillets. The great 

 motor tracts from the anterior central gyrus of the cortex reach 

 the medulla at the fourth month. Growing downward in the 

 neuroglia layer, on either side of the median line, they conceal the 



