DEVELOPMENT OF HIND-BRAIN DERIVATIVES. 



1 101 



FIG. 952. 



outward bending of the lateral walls of the brain-vesicle and the stretching and flattening of tin- 

 roof-plate. In consequence of these changes the roof of the rhombencephalon becomes reduced 

 to an attenuated sheet which, when viewed from 

 above, appears as a lozenge-shaped membrane 

 that closes in the subjacent cavity, the subse- 

 quent fourth ventricle. It has also been pointed 

 out (page 1049) that the relatively thick lateral 

 walls of the neural tube exhibit, even within the 

 cord-segment, a differentiation into a dorsal 

 and a ventral zone (the alar and basal lamina? 

 of His), which subdivisions are associated with 

 the sensory and motor root-fibres of the nerves 

 respectively. Similar relations, in a more pro- 

 nounced degree, are evident within the brain- 

 stem and are of much interest as indicating the 

 morphological correspondence of the purely 

 motor nerves (the third, fourth, sixth and 

 twelfth) on the one hand, and of the mixed 

 nerves (the fifth, seventh, ninth and tenth) on 

 the other. 



The Medulla. The great preponderance 

 of the nervous matter along the floor of the 

 fourth ventricle, as represented by the medulla, 

 is due primarily to the outward bending of the 

 lateral walls of the myelencephalon, supple- 

 mented by the accession of large tracts of 

 nerve-fibres that later grow in from other parts 

 of the cerebro-spinal axis. In consequence of 



the former change, the dorsal zones of the side-walls are gradually displaced laterally ; at 

 the same time they become partly folded on themselves to produce along their outer margin 

 the rhombic lip (His), which is directly continuous with the expanded and thin roof-plate. 

 Later, the dorsal zones come to lie almost horizontally, their ventricular surface corresponding 

 with that of the ventral laminae, in conjunction with which the floor of the definitive fourth 



Mid-brain 



Right hemisphere 

 Inferior colliculus 



Roof-plate 



Cerebellum 

 Cavity of 

 hind-brain 

 Lateral recess 

 Rhombic lip 

 Attachment of 

 roof 



Medulla 



Reconstruction of brain of human embryo of 22.8 

 mm., showing hind-brain and part of mid-brain viewed 

 from behind. X 12. Drawn from model made by 

 Dr. Ewing Taylor. 



Pineal body 



FIG. 953. 



Superior colliculus 



Cavity of i 



Inferior colliculus 



Corpus striatum 

 Cut 



Worm of 

 cerebellum 



Hemisphere of 

 cerebellum 



Lateral recess 

 Cavity "f hind-brain 

 (IV ventricle) 



Roof of hind-brain, lower part 



Reconstruction of hind-brain of human embryo of about three months 

 (50 mm.), viewed from side and behind. Drawn from His model. 



ventricle is later formed. Coin- 

 cidently with the outward mi- 

 gration of the dorsal laminae, 

 the ventral zones also thicken 

 and assume a much more hori- 

 zontal position, with their inner 

 ends separated superficially by 

 a median furrow and, deeper, 

 by the compressed remains of 

 the floor-plate. Very early and 

 before the flattening out of the 

 myelencephalon has advanced 

 to any marked extent, the de- 

 marcation between the dorsal 

 and ventral zones is evident as 

 a lateral longitudinal groove 

 on the ventricular surface of 

 the myelencephalon. Indica- 

 tions of this division persist 

 and in the adult medulla are 

 represented by the fovea in- 

 ferior and the sulcus lateralis 

 seen on the floor of the fourth 

 ventricle. As in the cord-seg- 

 ment, so in the myelencepha- 

 lon the lateral walls are the 

 only regions of the neural tube 

 in which neuroblasts are devel- 



oped, the roof-plate and the floor-plate containing spongioblasts alone. 



Very early and before the flattening out of the myelencephalon has advanced to any marked 

 extent, within the ventral zones and close to the mid-line, appear groups of neuroblasts, from 

 which axones grow ventrally to form the root-fibres of the motor (hypoglossal) nerves. Sensory 



