524 



TEXT-BOOK OF EMBRYOLOGY. 



In the region cf the auditory segment an important neurone group appears 

 which is possibly a differentiation of the extreme dorso-lateral portion of the 

 basal plate. This is Belters' nucleus, which apparently receives vestibular 

 and cerebellar fibers and sends uncrossed descending bundles along the outer 

 lateral part of the reticular formation and also ascending and descending crossed 

 and uncrossed fibers along its outer mesial portion (part of the medial longi- 

 tudinal fasciculus). This nucleus thus represents, apparently, like the nucleus 

 ruber and nucleus of Darkschewitsch (below), a differentiated portion of the 

 intersegmental neurones in especial connection with suprasegmental efferent 

 fibers which thereby act on many brain and cord segments. 



The great development of the reticular formation here and caudally possibly 

 causes a ventro-lateral displacement of the contained nucleus ambiguus and 

 efferent facial nucleus and consequently the arched or hook-shaped course of 



Genu facialis 



-Forward 



© 



med sulcus 

 A 



med.sulcus 



B 



medsulcU5 



Fig. 450. — Diagram illustrating the development of the genu of the facial nerve in the human 

 embryo. The drawings show the right facial nerve and its nucleus of origin, in three stages: 

 the youngest, A, being a 10 mm. embryo, and the oldest, C, a new-born child. The relative 

 position of the abducens (VI) nerve is represented in outline; its nerve trunk is not shown, as 

 the structures represented are seen from above. Slreeter. 



their root fibers as seen in transverse section (Streeter) . At the same time, the 

 nucleus of the VI, which originally was caudal to the YII, migrates cranially, 

 carrying the facial efferent roots with it. This gives rise to the genu facialis 

 (Streeter, Fig. 450). 



In the mid-brain (Fig. 451), what appears to represent the basal plate 

 forms an eminence, the tegmental swelling. Later there is differentiated from 

 this the reticular formation of this region, containing various nuclei and 

 traversed by radial, longitudinal and arcuate fibers, many of the latter arising 

 from the later differentiating dorsal portions (corpora quadrigemina) of the 

 lateral mid-brain walls. An important neurone group of the reticular forma- 

 tion system which appears in this region is the nucleus of Darkschewitsch. Its 

 descending axones form a part of the medial longitudinal fasciculus and 

 probably appear at the end of the first month. The nucleus ruber is probably 

 differentiated from the forward extremity of the tegmental swelling which over- 

 laps into a prechordal region (Fig. 463). Its axones (crossing asForePs decus- 

 sation and forming the rubrospinal tract) probably develop early. This 



