ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 655 



The guanin granules are moved forwards. There is an intricate 

 protoplasmic mechanism. 



In darkness the cones are almost inaccessibly inclosed by the 

 tapetum-inass, while the rods are freely exposed and have behind them 

 the reflecting guanin-mass. The rods are the essential percipient 

 elements in twilight vision. 



In the day the cones are contracted, apposed to the membrana limi- 

 tans externa, fully exposed to the light. Behind them lies the tapetum 

 darkened with mixed pigment, which, like the chorioideal pigment in 

 other animals, hinders the diffusion of the light, and increases localisa- 

 tion at the expense of intensity. The rods are unrecognisably imbedded 

 in the opaque mass of fnscin and guanin, hardly accessible by the light. 

 The cones are the essential percipient elements of broad daylight vision. 



Histology of Optic Nerve.* — J. T. Graydon has carried out some 

 researches on the origin and development of the epiblastic trabecule 

 and the pial sheath of the optic nerve of the frog. Illustrations are 

 also given of variations in the epiblastic trabecules in the developing 

 optic nerve of mouse, chick, trout, and dogfish. It is shown that the 

 trabecular are entirely epiblastic in origin, and that the nerve fibres 

 lie, throughout the whole of their course, in the optic stalk, within the 

 membrana limitans externa, on the outside of which the connective- 

 tissue layer of the pial sheath is gradually formed. Three functions are 

 performed by the cells of the optic stalk, viz. : (1) they conduct the 

 nerve fibres, which in their turn resolve the constitution of the cells of 

 the stalk so that they (2) provide the nerve fibres with a supporting 

 framework, which (3) provides the whole interior of the optic nerve 

 with an elaborate system of minute lymph channels. 



Histological Changes in Pancreas. f — S. Tschassownikow has tried 

 the effect of binding the pancreatic ducts as a contribution to the solution 

 of the problem of the inlets of Langerhans. The gland tubuli perish and 

 the islet cells are undoubtedly preserved. There is alteration of their size 

 and form, which is, however, dependent on the growth of the connective 

 tissue. Further, it is asserted that the islets are incapable of being 

 transformed into zymogen-containing elements as so many authors 

 affirm. 



Canals of Glandular Epithelium.^— W. Rubaschkin has studied 

 the submaxillary gland, the glands of the stomach, and the pancreas 

 with reference to the intracellular ducts, the so-called secretory capilla- 

 ries. These clear ducts are transitory variable appearances due to the 

 coalescence of vacuoles containing fluid secretion. 



Epidermis of Lepadogaster.§ — F. K. Studnicka describes the 

 gland cells — " saccular serous glands " and " mucous cells " — in the 

 epidermis of this Teleostean, and the quite unique cuticular structure of 

 the suctorial disk. 



* Quart. Joum. Micr. Sci , 1. (1906) pp. 479-92 (2 pis.). 

 t Arch. Mikr. Anat., lxvii. (1906) pp. 758-72 (1 pi.). 

 X Anat. Anzeig., xxix. (1906) pp. 209-16 (6 figs.). 

 § Tom. cit., pp. 132-44 (12 figs.). 



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