DEVELOPMENT OF THE NERVOUS SYSTEM' 



739 





a great extent its epithelial nature, is expanded in a lateral direction. Later, by 

 the growth and backward extension of the cerebellum, the roof is folded inward 

 toward the cavity of the fourth ventricle; it assists in completing the dorsal wall 

 of this cavity, and is also invaginated to form the ependymal covering of its choroid. 

 plexuses. Above it is continuous with the posterior medullary velum; below, with 

 the obex and ligulse. 



The development of the medulla oblongata resembles that of the medulla spinalis, 

 but at the same time exhibits one or two interesting modifications. On transverse 

 section the myelencephalon at an early stage is seen to consist of two lateral walls, 

 connected across the middle line by floor- and roof-plates (Figs. 647 and 648). 

 Each lateral wall consists of an alar and a basal lamina, separated by an internal 

 furrow, the remains of w r hich are represented in the adult brain by the sulcus 

 limitans on the rhomboid fossa. The contained cavity is more or less triangular 



Tania 

 Rhombic lip 



V. N. Motor root 

 V. N. Sensory root 



Oanglia of VII. and 

 VIII. Ns. 



Auditory vesicle 



FIG. 649. Hind-brain of a human embryo of three 

 months viewed from behind and partly from left side. 

 (From model by His.) 



FIQ. 650. Exterior of brain of human embryo of four 

 and a half weeks. (From model by His.) 



in outline, the base being formed by the roof-plate, which is thin and greatly 

 expanded transversely. Pear-shaped neuroblasts are developed in the alar and 

 basal laminae, and their narrow stalks are elongated to form the axis-cylinders of 

 the nerve fibers. Opposite the furrow or boundary between the alar and basal 

 laminae a bundle of nerve fibers attaches itself to the outer surface of the alar 

 lamina. This is named the tractus solitarius (Fig. 648), and is formed by the sensory 

 fibers of the glossopharyngeal and vagus nerves. It is the homologue of the oval 

 bundle seen in the medulla spinalis, and, like it, is developed by an ingrowth of 

 fibers from the ganglia of the neural crest. At first it is applied to the outer surface 

 of the alar lamina, but it soon becomes buried, owing to the growth over it of the 

 neighboring parts. By the fifth week the dorsal part of the alar lamina bends 

 in a lateral direction along its entire length, to form what is termed the rhombic 

 lip (Figs. 648, 649). Within a few days this lip becomes applied to, and unites 



