THE NEURAL TUBE AND ITS DERIVATIVES 



35 



mesencephalon and rhombencephalon into the spinal cord. The thalamus is 

 produced by a thickening in the alar lamina and is separated from the hypo- 

 thalamus by the sulcus limitans, which can be traced as far as the optic recess 

 rostral to the ridge produced by the optic chiasma. 



The hypothalamus 1 represents the basal lamina and gives rise to the tuber 

 cinereum, posterior lobe of the hypophysis, and the mammillary bodies. From the 

 dorsal edge of the alar lamina, where this is attached to the thin roof plate, there 

 is developed a thickened ridge, the epithalamus, which is transformed into the 

 habenula and the pineal body. The roof plate of the diencephalon remains 

 thin and forms the epithelial lining of the tela chorioidea or roof of the third 

 ventricle. 



The Mesencephalon. The basal plate of the mesencephalon thickens to 

 form the cerebral peduncles (Fig. 17), the alar plate forms the lamina quad- 

 rigemina in which are differentiated the quadrigeminal bodies; the cavity be- 

 comes the cerebral aqueduct. 



TABLE SHOWING SUBDIVISIONS OF THE NEURAL TUBE AND THEIR DERIVATIVES (Modified from a 

 Table in Keibel and Mall, Human Embryology). 



1 The pars optica hypothalami, including the optic chiasm, is, properly speaking, not a part 

 of the hypothalamus at all, but belongs to the telencephalon (Johnston, 1909, Jour. Comp. Neur , 

 vol. 19, and 1912, Jour. Comp. Neur., vol. 22). 



