DEVELOPMENT OF HIND-BRAIN DERIVATIVES. 



IIOI 



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

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

 to an attenuated sheet which, when viewed from 



Ft'. 952. 



Mid-brain 



Kight hemisphere 



Inferior coUiculus 



Roof-plate 



Cerebellum 

 Cavity of 

 hitid-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. • 12. Drawn from model made by 

 Dr. Ewing Taylor. 



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-flbres 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 \entricular surface corresponding 

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



Pjq gc, ventricle is later formed. Coin- 



superior coUiculus cideutly 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 coinpressed remains of 

 the floor-plate. \'ery 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 



Pineal body 



brain 



Inferior coUiculus 



Superior 

 medullary \eluin 



Hemisphere of 

 cerebellum 



Corpus striatum 



Cut wall of cerebral 

 hemisphere 



Pontine flexure 

 Lateral recess 



Cavity of hmd-brain 

 (IV ventricle} 



Roof of hind-brain 



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

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



