580 SUMMARY OF CURRENT RESEARCHES RELATING TO 



in its variations in groy and white substance, and in different regions of 

 these. Tho firmness of tho glia-cclls seems to vary inversely with 

 that of tho nerve-cells. Inter alia Prof. Gierke notes that the nerve- 

 fibres are everywhere enveloped in an ensheathing neuroglia network, 

 whoso knots represent glia-cells, and the threads processes. The 

 main threads are bound together by glia-fibres, and tho resulting net- 

 work nerve-fibre sheath is of extreme fineness. He maintains that 

 tho medulla-containing nervc-fibrcs are never imbedded directly in 

 the matrix, but are always separated from it by the formed elements 

 of the neuroglia. The glia framework of the white substance is 

 formed as usual from cells and matrix, but nerve-fibres sometimes 

 occupy the meshes of the network. Tho arrangement and varying 

 quantitative development of the elements, their relation to the blood- 

 vessels which accompany the stronger strands, &c, are next discussed. 

 The quantitative development of the neuroglia in the white substance 

 of tho spinal cord is proportionately less in the lower vertebrates 

 than in mammals, but Dr. Gierke is unable yet to formulate any 

 certain law. The memoir ends with a comparison of the development 

 of neuroglia in different regions of the white substance. To this 

 most elaborate research, dealing specially as yet only with tbe sup- 

 porting substance, a continuation is promised, which, if as thorough 

 as the above, will go far to justify the author's assurance that the 

 central nervous system " which has been hitherto so divergently 

 described and in its essential nature really so little known, will 

 henceforth be one of the best known tissues of the body." 



y. General.* 



Parietal Eye of Hatteria.j — Mr. W. Baldwin Spencer reports a 

 remarkable discovery — the presence of a median parietal, or as it 

 might more justly be called interparietal, eye in Hatteria punctata, the 

 curious lizard of New Zealand. The epiphysis cerebri of amphibians 

 and reptiles becomes divided into two parts, the proximal of which 

 remains connected with the brain, while the distal is a bladder-shaped 

 structure. In Anguis fragilis this distal part, as Von Graaf finds, loses 

 all connection with the brain, and developes into a structure resembling 

 a highly organized invertebrate eye ; no nerve, however, is connected 

 with it. In Hatteria the similar eye-like organ is provided with a 

 well-marked nerve. The eye is enclosed in a capsule of connective 

 tissue ; anteriorly there is a lens which forms the anterior boundary 

 of a vesicle, the walls of which are formed from within outwards of 

 the following layers : — (1) a not well-marked layer, (2) a layer of 

 rods imbedded in dark-brown pigment, (3) a double or triple row of 

 nuclei, (4) a clear layer which may be called the molecular, and (5) a 

 layer of nuclei two or three rows deep. The nerve which enters the 

 eye posteriorly spreads its fibres round the vesicle. A blood-vessel 

 ramifies in the surrounding connective tissue. The eye lies exactly 

 in the median line, and the nerve is single ; the latter appears to 

 represent the stalk connecting the distal with the proximal outgrowth 

 from the thalamencephalon. The eye does not reach the surface, but 



* This section is limited to papers which, while relating to Vertebrata, 

 have a direct or indirect bearing on Invertebrata also, 

 t Nature, xxxiv. (1886) pp. 33-5 (2 figs.). 



