THE NERVOUS SYSTEM. 



521 



— the foramina of Luschka. The roof of the fourth ventricle, where present, 

 is thus composed of an inner ependymal epithelium — the expanded roof plate 

 of the neural tube — and an outer mesodermal covering containing blood vessels. 

 Other gross changes chiefly involve the basal plate. At the beginning of the 

 fifth week this does not much exceed the alar plate in thickness and is separated 

 from the opposite basal plate by an inner median sulcus (Fig. 452). The basal 

 plate now increases in thickness and thereby both deepens the sulcus and con- 

 tributes to a flattening out of the lateral walls, so that all portions by the sixth 

 week lie approximately in .the same horizontal plane (Fig. 454). Later, the 

 floor plate increases in thickness more rapidly and the sulcus becomes shallower 

 (eight weeks) (Fig. 455). The band of vertical ependyma fibers passing through 



Mesencephalon 



Epiphysis.] 

 Diencephalon 



- Isthmus 



. Cerebellum 



• Transverse fold 



-Rhombic lip 



Olfactory lobe 

 Optic stalk 



Infundibulum Hypophysis Basilar artery 



Fig. 448. — Lateral view of a model of the brain of a 7J weeks' (18.5 mm.) human embryo. Bis. 



it is the septum medulla. It is bounded on each side by a vertical extension of 

 the marginal layer which for convenience will be referred to as the septal 

 marginal layer (Figs. 453, 454 and 455). 



The histological condition of this part of the tube at the beginning of five 

 weeks has already been described. The lateral walls consist of an inner layer 

 of closely packed cells, of a mantle layer consisting of efferent neurones and a 

 simple system of intermediate neurones, and an outer marginal layer containing 

 the longitudinal bundles of incoming afferent roots and longitudinal axones of 

 intermediate neurones (see p. 511). It has been seen that this condition has 

 been brought about by the proliferation of cells near the tube cavity, which 

 migrate outward, at the same time many of them differentiating into neuro- 

 blasts and nerve cells and thereby forming the mantle layer. As in the cord, 

 the basal plate takes the lead and thus at first outstrips the alar plate, as shown 



