DEVELOPMENT OF HIND-BRAIN DERIVATIVES. 



1103 



duplicature, directed towards the brain cavity, the mesoblast grows and later develops blond- 

 vessels, and is converted into a vascular complex that eventually forms the choroid plexus of the 

 fourth ventricle. From the manner of its development, it is evident that the plexus is excluded 

 by the ependymal layer from the ventricular space, outside of which the pial blood-vessels, 

 therefore, really lie. The conversion of the upper part of the primary velum into the thicker 

 definite inferior medullary velum follows the addition of nervous substance during the develop- 

 ment of the cerebellum. Similar thickening of the roof-sheet at the lower angle of the ventricle 

 results in the production of the obex and the taenise. 



The Pons. The pons arises as a thickening of that part of the metencephalon which forms 

 the anterior wall of the pontine flexure. In its essential phases the development of the pons 

 probably closely resembles that of the medulla, since the early metencephalon presents the same 

 general features as does the myelencephalon. Thus, the ventral zones of its lateral walls play 

 an active role in the production of the tegmental portion of the pons and the nuclei of origin of 

 the motor root-fibres of the fifth, sixth and seventh nerves, whilst the floor-plate becomes the 

 raphe. In addition to providing the reception-nuclei of the sensory cranial nerves, and, per- 

 haps, the pontine nuclei, the dorsal 



zones contribute the neuroblasts which p IG> 955- 



become the nervous elements of the c 



cerebellum. As in the medulla, so in 

 the pons the great ventral tracts are 

 secondary and relatively late additions 

 to the tegmentum, which must be re- 

 garded as the primary and oldest part 

 of this segment of the brain-stem, the 

 bulky ventral nervous masses taking 

 form only after the appearance of the 

 cerebro-spinal and cerebro-cerebellar 

 paths. In a manner analagous to that 

 by which the sensory part of the vagus 

 is at first loosely applied and later in- 

 corporated with the medulla, the sen- 

 sory fibres of the trigeminus are for a 

 time attached to the surface of the 

 dorsal zone of the pons, subsequently 

 becoming covered in and more deeply 

 placed by the addition of peripheral 

 tracts. Likewise the fibres of the audi- 

 tory nerve come into relation with the 

 superficially situated reception-nuclei 

 of "the cochlear and vestibular nerves. 



The Cerebellum. The develop- 

 ment of the human cerebellum pro- 

 ceeds from the roof-plate and adjacent 

 parts of the dorsal zones of the lateral 

 walls of the metencephalon. In an 

 embryo 22.8 mm. long, the cerebellar 

 anlage consists of two lateral plates 



connected by a narrow thin intervening lamina representing the roof-plate (Fig. 952). After 

 the apposition of the lateral plates, which soon occurs, this bridge disappears, the developing 

 cerebellum for a time appearing as an arched lamina enclosing the upper part of the cavity of 

 the hind-brain (Kuithan 1 ). 



The subsequent development of the human cerebellum has been recently carefully studied 

 by Bolk 2 in a series of about forty fcetuses, hardened in formalin and ranging from 5 to 30 cm. in 

 their entire (crown-sole) length. The following account is based largely on these investigations. 

 In a fcetus of 5 cm., about nine weeks old, the cerebellar anlage is represented by a horseshoe- 

 shaped thickening of the metencephalic roof, the cerebellar lamina, whose upper margin is con- 

 nected by a conspicuous fold with the mid-brain and whose lower border has attached to it the 

 primary velum the thin rhomboidal roof-plate of the myelencephalon. Median sagittal section 

 of the cerebellar lamina at this stage (Fig. 955, A) shows its form to be asymmetrically biconvex, 

 the more convex surface encroaching upon the brain-cavity. In a slightly older fcetus ( Fig. 

 955, ) the cerebellar lamina has become triangular, in section presenting a superior, an 

 anterior, and an inferior surface. From its attachment along the superior margin of the lamina 

 the inferior velum dips forward toward the pontine flexure and, forming a transversely cresentic 



1 Miinchner med. Abhand., 1895. 

 *Petrus Camper, 36 Deel, 1905. 



Median sagittal sections showing four early stages of develop- 

 ment of human cerebellum, from fcetuses from 5 to 9 cm. long; 

 nib, mid-brain ; c, cerebellum ; sv, iv, superior and inferior medul- 

 lary velum ; vc, ventricular cavity ; d, cavity of diencephalon ; p, 

 pons ; m, medulla ; s, spinal cord ; if, incisura fastigii ; /, sulcus 

 stnodularis. (Drawn from figures of 



primarius ; j>, sulcus post 



of Bolk.) 



