ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 165 



nucleus, but not otherwise. The form is conditioned by a marginal fold 

 of the membrane, which is fixed by acid re-agents, whereas in very weak 

 salt solution it is not fixed, and allows the disk to swell up into a sphere. 



Enamel Prisms.* — Ernst Smreher has investigated the enamel 

 prisms and the cementing substance of the enamel in human teeth. The 

 enamel fibres are not as a rule prismatic hexagonal structures, but pillars 

 of irregular form. The author explains how the form arises in the 

 course of development. 



Amitosis and Multinucleate Cells in Epithelium.t — M. Pacaut 

 has found, especially in Eodents, the frequent occurrence of multi- 

 nucleate cells in many-layered epithelium, e.g. of the oesophagus. The 

 nuclei (2-4) usually remain apposed ; they are formed by amitotic 

 division, which is in most cases entirely normal. 



Odontoclasts.! — Coyne and Cavahe describe giant multinucleate 

 phagocytic cells which help in the destruction of teeth. On the decay- 

 ing surface of a tooth affected by caries, the fragments of enamel and 

 ivory are seen to be surrounded by a finely granular clear envelope com- 

 posed of these odontoclasts. Outside these there is a cellular zone of 

 uninucleate, small, embryonic cells, which give rise to the odontoclasts. 

 A microbic infection may be the "primum movens," but the actual 

 destruction and absorption are due to the odontoclasts. 



c Q-eneral. 



Poecilogony.§ —A. Giard has collected and discussed instances of 

 what he calls pcecilogony, that is, the occurrence in identical or nearly- 

 related species of quite distinct modes of development, which are usually 

 correlated with different conditions of life. 



Cranial Nerves in Petromyzon.|| — J. B. Johnston has investigated 

 the components of these nerves in the Ammoccetes of P. dorsatus. In 

 the arrangement of the cranial nerve-components Petromyzon agrees in 

 general with fishes, but shows some marked primitive characters. One 

 or two of the special features demonstrated may be quoted. The 

 profundus nerve retains its independence, but has added to it the 

 equivalent of the rami ophthalmici superficiales trigemini and facialis of 

 fishes. The visceral sensory component is very small in the facialis, but 

 large in the glossopharyngeus and vagus. A sympathetic trunk and 

 ganglia are present in the head. 



Head-Ganglia and Sensory Line in Selachia.f — W. Klinkhardt 

 has studied the development of these in Spinax niger. He describes 

 the ectoderm folds, and the origin therefrom of the sensory lines of 



* Arch. Mikr. Anat., lxvi. (1905) pp. 312-31 (3 pis.). 



+ Verh. Anat. Ges., xix. Vers. See also Anat. Anzeig., xxvii. (1905) Erg. Heft, 

 pp. 144-5. 



X Loc. cit. See also Anat. Anzeig., xxvii. (1905) Erg. heft, p. 205. 

 P§ Bull. Scient., xxxix. (1905) pp. 153-87. See also Zool. Zentralbl., xii. (1905) 

 p. 773. 



|| Morphol. Jahrb., xxxiv. (1905) pp, 149-203 (1 pi. and 18 figs.). 



^ Jena Zeitschr., xxxiii. (1905) pp. 423-86 (3 pis. and 6 figs.). 



