NEUROMERES AND METAMERES 299 



The comparison of figures 3 and 4 suggests that there has been an ontogenetic 

 migration ('neurobiotaxis' Uriens-Kappers) of the oculomotor nidulus toward 

 the lumen of the midbrain. Multipolar ganglion cells (cl.R.B.) have appeared 

 in the dorsal wall of the midbrain near the median line and the ependymal lining. 

 The neuraxon processes of these cells enter the mandibular branch of the tri- 

 geminal nerve ( see figs. 1 and 2). 



Fig. 5 A frontal section of an 18 to 19-mm. Squalus embryo in the region of 

 the isthmus, showing the trochlearis nidulus and dorsal chiasma. At this stage 

 most of the trochlearis nidulus lies within the first (cerebellar) rhombomere, but 

 also extends anteriad into the region of the midbrain. Since, however, the 

 larger number of the neuroblasts of the trochlearis are situated in rhombomere 

 1, the nerve is clearly a hindbrain nerve and must be assigned to that part of the 

 brain in schemes of metamerism. 



Fig. 6 A cross-section of a Squalus embryo ('pup' stage) in the region of the 

 isthmus and of the trochlearis chiasma (ch. dors. IV). As a result of the dis- 

 placement anteriorly, the trochlearis nidulus does not appear in cross-sections 

 of the brain at this stage. The section, however, shows some of the trochlearis 

 neuraxons (a.v.tr'ch.) in the course of their ascent from a fiber tract lying near 

 the median ventral sulcus of the floor of the midbrain. These may be traced in 

 subsequent sections to their union with the dorsal chiasma and in the floor of 

 the midbrain to the central nidulus which, through a forward shifting of the 

 ventral wall of the brain, now appears as the floor of the midbrain region. 



Fig. 7 A parasagittal section of an' 18-mm. Squalus embryo, showing the 

 relative positions of the oculomotorius and trochlearis niduli. The position of 

 the trochlearis chiasma in the dorsal wall of the brain (ch.do7-s.IV.) marks the 

 division between midbrain and hindbrain vesicles. The lateral recess (rec.) 

 in the floor of the brain lies within the limits of the midbrain vesicle and does 

 not indicate a boundary between the primary brain divisions. This fold in the 

 lateral wall makes the niduli of the oculomotor and trochlearis nerves appear in 

 the section to be more distinct than they actually are by bending the somatic 

 motor column laterally out of the plane of the section. Most of the trochlearis 

 nidulus lies well within the bounds of the hindbrain (first rhombomere). The 

 elongated nidulus of the oculomotorius is only partially shown in the section. 



Fig. 8 A parasagittal section of a Squalus embryo in the region of the mid- 

 brain ('pup' stage). At this stage, through the displacement of the floor of the 

 brain, the trochlearis nidulus appears to lie well within the limits of the mid- 

 brain, in close proximity to the nidulus of the oculomotorious. Their relations 

 to the lateral recess resemble those seen in the earlier stage (fig. 7). The bound- 

 aries of the two niduli, however, are not very distinct, and it is possible to trace 

 neuraxon processes of neuroblasts anterior to the recess posteriorly and dorsally 

 toward the trochlear chiasma. In other words, the niduli of the two nerves 

 overlap each other, schemes of metamerism notwithstanding. The Rohon- 

 Beard cells are conspicuous in the dorsal wall of the brain. All figures are based 

 on camera drawings of embryos of Squalus acanthias prepared by the Ranson- 

 Cajal method. Abbreviations: Figures 3 to 8. 11,111, Second and third neuro- 

 meres (Neal, '96, '98); ax.tr'ch., axons of the trochlearis; cl.R.B., Rohon-Beard 

 cells of midbrain; isth., isthmus; midl.oc, nidulus of the oculomotorius; nidi, 

 tr'ch, nidulus of the trochlearis; oc, oculomotorius nerve; rec, recess in floor of 

 midlirain; tr'ch., trochlearis nerve. 



