THE NERVOUS SYSTEM. 523 



The development of these nuclei is not fully known, but they are derived from 

 the alar plate, except possibly Deiters' nucleus (see p. 517), the nuclei of the 

 later formed cochlear nerve occupying the more external position. The ves- 

 tibular nuclei apparently send axones both to cerebellum and reticular formation. 

 The cerebellum itself may be regarded as primitively a receptive vestibular 

 structure (p. 466) and probably receives vestibular root fibers. The axones 

 of the cochlear nuclei pass across the median line, along the ventral border of 

 the reticular formation (second half of second month), forming the trapezium. 

 On the lateral boundary of the opposite reticular formation they ascend, form- 

 ing the lateral fillet, to the suprasegmental posterior corpus quadrigeminum. 

 Accessions are received from the superior olive, in which some of the trapezium 

 fibers terminate. 



The alar plate of this segment also forms the substantia gelatinosa and the 

 anterior portions of the olivary nuclei in this region. The various remaining 

 tracts assume the same positions as further caudally. 



Later, the pyramids are added ventrally to the fillet, and the great develop- 

 ment of the pons leads to its covering the ventral surface of part of this region. 

 Owing to the late development of the pons and pyramids, the trapezium is thus 

 uncovered and lies on the ventral surface of the rhombic brain during the third 

 month. It is permanently uncovered in the dog and cat. 



In the trigeminus segment, the terminal nucleus of the afferent portion of 

 this nerve is probably similarly formed from the alar plate. Its axones decus- 

 sate, probably joining the fillet, and proceed to the thalamus, which is connected 

 with the pallium. Descending axones from cells in the mid-brain roof form 

 part of the trigeminus known as its descending or mesencephalic root. The 

 view has been advanced (Meyer, Johnston) that these are afferent neurones 

 equivalent to certain dorsal horn cells found in some adult and embryonic 

 Vertebrates and representing spinal ganglion cells which have become included 

 in the neural tube instead of becoming detached with the rest of the neural crest 

 (compare p. 452). 



In front of the lateral recess another extensive development of the alar plate 

 occurs, evidenced by the large rhombic lip of this region. The neuroblasts 

 thus differentiated form the enormously developed pontile nuclei whose axones 

 pass across the median line (fifth month) to the opposite cerebellar hemisphere, 

 forming the middle cerebellar peduncle or brachium pontis. The pons extends 

 over the ventral surface of the cephalic part of the medulla and over the ventral 

 surface of part of the mid-brain. It receives fibres from various parts of the 

 neopallium, which form a great part of the pes pedunculi or crusta. A still 

 greater development of the alar plate forms the cerebellum. 



In the mid-brain region, the reticular formation already described (p. 517) 

 is enveloped ventrally and laterally by the upward extension of the medial and 



