624 SUMMARY OF CURRENT RESEARCHES RELATING TO 



as sucli take no part in it. The spermatozoa of the second and third 

 stages remain as bundles in the protoplasmic mass arising from the 

 altered cells, and lie among the neighbouring families ; with the empty, 

 folded mother-cell-membrane they form the spermatoblasts of v. Ebner. 

 The spermatozoa are pushed out by the extension of the neighbouring 

 cell. After the spermatozoa of the third stage are completed, the cell- 

 families become regenerated from the stem-cells. 



Embryonic Cell-division.* — The most important results of Herr 

 E. Schwarz's investigations into cell-division in the embryo are: — 

 (1) The nucleus of cleavage-cells is a radial, uniaxial, " heteropolar " 

 structure. (2) The poles of division are separately laid down ; the axis 

 of division is perpendicular to the nuclear axis; the planes of division 

 are symmetrical, and the succeeding planes of division, as well as the 

 axes, are perpendicular to one another, (3) The chromatin-structures 

 are well-developed loops, the number of which may be twenty-four. 

 (4) The nuclear spindle consists of two kinds of achromatic bundles of 

 fibres, both of which arise from the nucleus, and both parts pass into 

 daughter-nuclei. (5) The chromatic daughter-loops each form nuclear 

 elements with the achromatin which belongs to them, and by their union 

 the daughter-nuclei are formed. (6) Direct division does not obtain 

 either in the cellular or in the plasmodial portion of the germinal disc. 



j3. Histologryf 



Structure of Nerve-fibres.| — Prof. F. Leydig thinks it opportune to 

 direct attention to some of the conclusions to which he has been led 

 with regard to the structure of nerve-fibres. He has already given 

 reasons for believing that the nerve-fibres of Annelids and Arthropods 

 are better called tubes, and that they consist of a spongioplasm enve- 

 loping a hyaloplasm, the latter being the true nervous material, and the 

 former a supporting network. 



Among Vertebrates the non-medullated trunks of the olfactory nerve 

 have been examined in the Salamander and the Cat, and it has been 

 found that the " fibrils " are parts of a hollow system which is traversed 

 by a fine network. The author has pointed out that the more essential 

 constituent of a nerve — the material found in the ducts, which is pro- 

 bably of a semifluid nature — has been left out of consideration. Medul- 

 lated nerves have been chiefly examined in Hyla and Bana ; when 

 cross-sections of these, preserved in chromic acid, were examined, the 

 nerve-fibres were seen to be tubes filled with a clear substance ; this last 

 is traversed by lines of network which call to mind what have been seen 

 in the nerve-tubes of Invertebrates. The recent work of Joseph, who 

 has made more extensive observations on Vertebrates, substantiates the 

 conclusions of Prof. Leydig. Eetzius, also, in essential points supports 

 Leydig. 



The recognition of the fact that in the minute structure of ganglionic 

 spheres and of nerve-fibres we have to distinguish a supporting substance 

 (i-pongioplasm) from an inclosed homogeneous material (hyaloplasm 

 or true nervous substance) is closely connected with the results which 

 tbe author has obtained with regard to muscular tissue. There, too, we 



* MT. Embryol. Inst. K. K. Univ. Wien, 1888 (1889) pp. 30-73 (2 pla.). 

 t This section is limited to papers relating to Cells and Fibres. 

 X Biol. Centralbl., ix. (1889) pp. 199-204. 



