THE NEUKAL TUBE. 33 



their cells give off processes which become nerve- fibres. These fibres grow out 

 both from the dorsal and the ventral ends of the ganglia, and, together with the 

 ganglia, they form, in the cranial region, certain of the cerebral nerves, and, in 

 the spinal region, the posterior roots of the spinal nerves. 



The fibres which grow out of the dorsal ends of the ganglia enter the walls 

 of the neural tube, and by their means the ganglia regain connexion with the 

 tube. 



The fibres which grow out from the ventral end of each spinal ganglion unite 

 with the fibres of the corresponding anterior nerve-root, which, in the meantime, 

 has grown out from the cells of the ventral part of the lateral wall of the spinal 

 portion of the neural tube, and form with them a spinal nerve-trunk. 



The Differentiation of the Neural Tube. Before the neural groove is con- 

 verted into a closed tube, an expansion of its anterior part indicates the separation 

 of the neural rudiment into cerebral and spinal sections, the dilated portion being 

 the rudiment of the brain and un dilated part the rudiment of the spinal medulla. 



Whilst the cerebral portion is still unclosed, three secondary dilatations of its 

 walls indicate its separation into three sections, the primitive fore-brain, the 

 mid-brain, and the hind-brain ; the primitive fore-brain being the most cephalward 

 or anterior and the hind-brain the most caudal or posterior of the three (Fig. 38). 



Shortly after the three segments of the brain are defined, and before it becomes 

 a closed tube, a vesicular evagination forms at the cephalic end of each lateral 

 wall of the primitive fore-brain region. These evaginations are the primary optic 

 vesicles, and they are the rudiments of the optic nerves, the retinae, and the 

 posterior epithelium of the ciliary body and the iris of the eye -ball. 



When the cerebral portions of the neural folds meet and fuse dorsally the 

 cerebral dilatations become the primitive brain vesicles, each vesicle possessing 

 its own cavity and walls, but the cavities of the three vesicles are continuous with 

 one another, and the cavity of the hind-brain vesicle is continuous, caudally, with 

 the central canal -of the spinal part of the neural tube. 



After the primitive brain vesicles are formed, a diverticulum grows out from 

 the cephalic end of the primitive fore-brain vesicle. This is the rudiment of the 

 secondary fore-brain. Its cephalic end soon divides into two lateral halves, which 

 are the rudiments of the cerebral hemispheres of the adult brain (Fig. 45). 



After their formation the cerebral hemispheres expand rapidly in all direc- 

 tions. They soon overlap the primitive fore-brain and mid-brain (Fig. 63), and, 

 eventually, the hind-brain also, and each gives off from the cephalic end of its 

 ventral wall a secondary diverticulum, the olfactory diverticulum, which becomes 

 converted, later, into the olfactory bulb and olfactory tract. 



When they first appear the rudiments of the cerebral hemispheres are con- 

 nected together, across the median plane, by a part of the cephalic end of the 

 wall of the secondary fore-brain dilatation, which is called the lamina terminalis. 

 This primitive connexion between the two cerebral hemispheres persists through- 

 out the whole of life, and it is supplemented, at a later period, by the formation 

 of three secondary commissures, the corpus callosum and the fornix, which grow 

 across the space between the cerebral hemispheres and connect their medial walls 

 together, and the anterior commissure wjiich grows through the lamina terminalis 

 and connects the temporal portions of the two hemispheres. 



The Fate of the Walls of the Primitive Brain Vesicles. The primitive 

 hind-brain, which is also called the rhombencephalon, is separated in the later 

 stages of development into two parts. (1) A caudal portion which is connected 

 with the medulla spinalis, and which becomes the medulla oblongata or myelen- 

 cephalon of the adult brain. (2) A cephalic portion which is continuous at one 

 end with the medulla oblongata and at the other with the mid-brain. The ventral 

 wall of the cephalic portion of the primitive hind-brain is ultimately converted 

 into the pons, and its dorsal wall differentiates into two parts a caudal part 

 which becomes the cerebellum ; and a cephalic part which is converted into the 

 anterior medullary velum and the brachia conjunctiva. The brachia conjunctiva 

 connect the cerebellum with the ventral part of the mid-brain. The pons and 

 cerebellum form the metencephalon of the adult, whilst the brachia conjunctiva 



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