ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 57 



the longitudinal zones of the neural tube, viz. the basal and alar laniinie, 

 and the ganglionic crest. The primitive neural plate in the cat gives 

 rise both to the neural tube and to the ganglionic crest, the latter being 

 a derived and secondary element and not a co-ordinate intermediate 

 between the medullary plate and the somatic ectoderm. In the fore- 

 brain elements equivalent to the ganglionic crest are retained in the wall 

 at the vesicles and constitute the dorsal region of the neural plates, for 

 if the crest secondarily separates from the neural tube, in regions where 

 such separation fails to occur, it is more probable that the crest is 

 included in the brain than that it has been absolutely suppressed. 

 Accordingly the analysis of the prosencephalon is not to be attempted 

 in terms of the basal and alar plates alone, as has been customary since 

 His, but must include a dorsal or '* ectal " strip equivalent to the 

 ganglionic crest along its convexity, and this must include at least as 

 much of the brain w^all as lies ectal to the optic sulcus. This ganglionic 

 element (the primitive optic vesicle) at four somites forms the cephalic 

 extremity of the neural fold, and arches ventrally to the floor. The 

 optic vesicle and the ectoptic structures, whether above, in front of or 

 below the optic region, must then be considered of ganglionic equiva- 

 lency, a conclusion which entails a revision of His's analysis of the 

 brain. This the authors proceed to expound. 



Development of Olfactory Nerve.* — Charles Brookover emphasizes 

 the distinctness of the olfactory nerve from other nerves. It is more 

 closely allied in its development to taste-bud components of cranial 

 nerves, but its neurones remain in the ectoderm. It may be that an 

 early development of a protective nasal capsule has lessened the neces- 

 sity for withdrawal of the neurones from the outside. The development 

 of the nervus terminalis along with the olfactory nerve from the same 

 placode is unique among nerves that are somatic, sensory, or sympathetic. 

 The author refers in particular to his observations on turtle embryos. 



Development of Eye-muscles in Marsupials.!— Elizabeth A. Fraser 

 has studied the head-cavities and the development of the eye-muscles 

 in TricJiosN/'us vulpemla and some other Marsupials. Well-developed 

 premandibular head-cavities are characteristic of the Diprotodontiu ; 

 they are large in Trichosurus, Fhascolarctos, Phasrolomys and Macrofm. 

 In the Polyprotodontia, on the other hand, cavities may be present, as 

 for example in Perameles, but only in the form of comparatively small 

 UTegular spaces, quite insignificant as compared with those in the former 

 group, or they are altogether absent, as in Dasyurus, and probably also 

 in Didelpltyii The muscles from the first somite then develop ' from 

 a solid mesodermal mass as in higher Mammals, and not by proliferatioti 

 from the walls of the premandil)ular cavity. 



That some Marsupials have well-developed premandibular cavities, 

 not as yet observed in other Mammals, sliows their apartness, and recalls 

 the conditions existing in some reptiles. The walls of the premandibular 

 cavity, as in most other Vertebrates, give rise to the muscles innervated 



* Trans. Amer. Micr. Soc, xxxiv. (1915) pp. 7-20 (5 figs.), 

 t Proc. Zool. Soc, 1915, pp. 299-346 (2 pis. and 26 figs.). 



