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H. V. NEAL 



pressure of adjacent somites is very possible — in the opinion of 

 the writer, very probable — in tide case of the neuromeres in the 

 trunk region, where they develop in correlation with the devel- 

 opment of the mesodermic somites and later disappear as the 

 somites lose their rounded form and close contact with the wall 

 of the neural tube. The hindbrain neuromeres Crhombomeres'), 

 however, never show a regular alternation with the mesodermic 

 somites such as obtains in the trunk region, and, moreover, they 

 persist in the medulla long after all traces of mesodermic somites 



Fig. 1 A diagram of a horizontal section in the head region of a 13-mm. 

 embryo of Squalus acanthias, showing the neuromeres and their motor nerve 

 relations. The nidulus of the oculomotor lies in the somatic-motor column of 

 the midbrain. That of the trochlearis is continuous with that of the oculomotor, 

 but lies chiefly in the first (cerebellar) rhombomere. The motor fibers of the 

 trigeminal (ramus mandibularis trigemini) are connected with neuroblasts lying 

 within the second and third rhombomeres. The motor nidulus of the facialis is 

 remarkable, extending through four rhombomeres (rhombomeres 4 to 7). The 

 nidulus of the abducens extends through rhombomere 6 into the adjacent rhom- 

 bomeres 5 and 7. The motor nidulus of the glossopharyngeal nerve lies partly 

 in rhombomere 6 and partly in rhombomere 7, while that of the vagus lies pos- 

 terior to the nidulus of the glossopharyngeal in a region devoid of neuromeric 

 divisions at this stage of development. For the sake of clearness, somatic motor 

 niduli of the cranial nerves are shown in the left wall of the brain, while the 

 splanchnic motor niduli are shown only in the right wall. 



disappear. Furthermore, the rhombomeres appear in some em- 

 bryos, e.g., those of Elasmobranchs, as local paired thickenings 

 of the wall of the medulla (Neal, '98). The rhombomeres, 

 therefore, may not be adequately interpreted as the passive re- 

 sults of mechanical bending or pressure. Moreover, the fact 

 that the differentiation of a typical rhombomere (3, figs. 1 and 

 2) occurs independently of connection with a cranial nerve root, 

 proves conclusively that rhombomeres may not be interpreted 



