ZOOLOGY AND BOTANY, MICROSCOPY, ETC 15 



earlier stages of development. That mesenchymatous cells are added to 

 the growing nerve in more advanced stages to form the connective 

 tissue sheaths, seems probable. 



The cells of the motor nerve primordia have no genetic relations to 

 the neurofibrils or nenraxons. In other words, they are not " nerve- 

 cells " in von Apathy's sense, nor do they unite in chains to form the 

 neuraxons or neurofibrils with their sheaths. Whether or not they 

 participate in the formation of the sympathetic is an open question. 

 The evidence on the whole favours, but does not prove, the conclusion 

 that most of the cells of the sympathetic have their source in the dorsal 

 ganglia. That the cells of the motor nerve primordia in Squalus for 

 the most part form neurilemma cells, can be convincingly demonstrated. 

 Thus the phenomena of spinal motor nerve histogenesis in Squalus 

 support the conclusions of Kupffer, Bidder, His, Harrison, and Lewis. 



The author passes to the histogenesis of the oculomotor, trochlear, 

 and abducens nerves, and finds that it differs in no essential respect 

 from the histogenesis of spinal somatic motor nerves. This creates a 

 strong presumption that pre-otic and post-otic divisions of the Vertebrate 

 body are fundamentally alike. The evidence in favour of the view that 

 the oculomotor is a mixed nerve homologous with typical cranial nerves 

 such as the trigeminal is so unconvincing, while the evidence of its 

 histogenesis and its central and peripheral relations so strongly supports 

 the view that it is a somatic motor nerve, that the acceptance of the 

 latter view seems unavoidable. The trochlear and the abducens must 

 also be ranked along with somatic motor nerves. 



In the region of the fore-brain there is at least one inetamere, 

 serially homologous with the metameres of the trunk, but more cannot 

 be safely said. The second inetamere has the premandibular as myo- 

 tome, the mid- brain as neuromere, the ophthalmicus profundus as the 

 somatic sensory nerve, and the ciliary as sympathetic ganglion. The 

 secondary splitting of the pre-mandibular myotome into dorsal and 

 ventral moieties is evidently correlated with the development of the 

 eyeball. The facts do not warrant the supposition that the oculomotor — 

 the somatic motor nerve of this metamere— has a bimeric distribution. 

 No one has been able to demonstrate the required two motor niduli. 

 The prernandibular is a single somite. 



The third metamere has the mandibular for its myotome, and its 

 neuromere is the cerebellum, within which lies the nidulus of the trochlear 

 nerve, which is therefore the somatic motor nerve of the segment. The 

 trochlear nerve becomes connected with the ramus ophthalmicus super- 

 ficialis trigemini, the somatic sensory nerve of the metamere. There is 

 evidence of. a transient sympathetic primordium. While the chiasma of 

 the trochlear is an anomaly, it may be regarded as ccenogenetic, and its 

 existence does not invalidate the comparison of this metamere with a 

 trunk segment. The ramus mandibularis trigemini appears to be the 

 splanchnic motor element of this metamere. 



The fourth metamere contains the third or hyoid myotome and the 

 fourth neuromere (second hind-brain neuromere). To this segment 

 may be assigned as the somatic sensory nerve the major root of the 

 trigeminal in part. Since no neural crest is proliferated from this 



