106 



NERVOUS SYSTEM 



begun to be developed. This is probably the case in the human embryo also. 

 The optic vesicles are formed in the pig as pits on the spread- out neural laminae, 

 and in the human embryo they are seen as hollow outgrowths from the fore-brain 

 before the laminae have united. In an embryo at the beginning of the third week 

 (embryo of 3'1 mm.) the brain-tube shows an anterior and posterior dilatation 

 connected by a narrower portion which corresponds to a sharp flexure (cephalic 

 flexure) which has bent the anterior on the posterior segment. The hinder and 

 larger dilatation is named the hind-brain (rhombencephalon), the anterior the fore- 

 brain (prosencephalon), while the intermediate portion is termed the mid-brain 

 (mesencephalon). The terminal wall of the fore-brain as shown in His' model is 

 not yet completely closed, a foramen being left called the neuropore. From the 

 fore-brain the optic vesicles project as wide-open diverticula connected now by 

 narrower portions, the optic stalks. 



During the third and fourth weeks, as the primitive brain increases in length 

 and the embryo becomes more folded, two other bends appear. The first of these 

 occurs at the junction of the brain and spinal cord, and is due to the head being 



bent down on the trunk (fig. 141 A). 

 It is called the cervical flexure, and 

 its concavity is necessarily ventral 

 like that of the primary head bend 

 (fig. 141s). The second flexure in- 

 volves the rhombic brain, and is not 

 determined by the curving of the 

 embryonal axis. It is produced by 

 a doubling of the hind-brain on itself. 

 Its concavity is directed dorsally. and 

 the point of the bend corresponds to 

 the position of the future pons ; it 

 is hence called the pontine flexure 

 (fig. 141c). While these bends are 

 forming, the fore-brain, by the increase 

 of the cephalic flexure, is folded back 

 below the parts of the tube originally 

 behind it, so that its ventral aspect 

 becomes closely applied to the ven- 

 tral surface of the rhombic brain, 



and the space between them is reduced to a narrow cleft filled with the 

 mesenchymatous investment of the primitive brain. In this tissue the dorsum 

 sellae is afterwards formed. Owing to this increase in the cephalic flexure the 

 mid-brain comes to be the most anterior part of the brain-tube, and is for some 

 time a very prominent feature, although later it loses in prominence by the 

 greater development of cerebrum and cerebellum. As morphogenesis proceeds, 

 the walls of the brain- tube, thus sinuously curved, become thickened and expanded 

 in certain regions to form the several parts of the adult organ, while in other 

 situations they remain mere ependymal lamellae, which, becoming inflected into the 

 interior of the primitive vesicles and carrying vascular mesenchyme between their 

 layers, form the epithelium of the choroid plexuses. Instead of tracing the 

 development of the brain as a whole from stage to stage, it will be more 

 convenient to describe separately, through all the phases, the several parts 

 derived from each primary division of the tube. 



Rhombic brain (rhombencephalon). The hind-brain in its early stages 

 has the very characteristic feature that the ependymal roof -plate is greatly thinned 

 and expanded. At first full and convex, this membranous lamella has a triangular 



FIG. 145. MODEL OF THE BRAIN OF A HUMAN 

 EMBRYO OF 53 MM. (ELEVENTH WEEK) FROM 

 BEHIND, TO SHOW THE FOURTH VENTRICLE, 

 RESTIFORM BODIES, AND CEREBELLUM. (His.) 



