390 SUMMARY OF CURRENT RESEARCHES RELATING TO 



rest of the wall. The opithelial cells in the neighbourhood pass from 

 without inwards, and lio below the cord, where they gradually become 

 isolatod. The dorsal wall of the embryo is developed as in Amphioxw. 

 The epithelium which forms the upper wall of the cavity of the gastrula 

 gives rise to tho two halves of the mesoblast and to the notochord, just 

 as in Amphioxus. 



&. Histology.* 



Nervous System of Amphioxus-t — Dr. E. Eohde has made an investi- 

 gation into the structure of the nervous system of Amphioxus, and finds a 

 close resemblance betwoen it and that of the same system in the poly- 

 chffitous Sthenelais. In both there are certain nerve-fibres which are 

 remarkable for their great size, constant position, and enormous lengths. 

 These colossal nerve-fibres are the processes of colossal ganglion-cells, 

 which are placed at definite distances from one another, partly at the 

 anterior (though raro in the brain), and partly at the hinder end of the 

 central nervous system. These fibres, which run from before backwards, 

 break up into an unpaired median fibre (which is always the largest), and 

 paired lateral fibres, and they are connected by fine lateral branches with 

 the other nerve elements. In both forms the supporting tissue is of 

 ectodermal origin. 



Changes of Position of Nucleus.! — HerrO. Schultze describes some 

 of the changes of position observed in the nucleus, (a) Passive displace- 

 ments. — The nucleus may be pressed to the surface by accumulations of dif- 

 ferent substances, e.g. fat and mucus, (b) Active displacements. — These are 

 associated with the maturation and fertilization of the ovum. Auer- 

 bach has described the rotation of the nucleus in the fertilization of 

 Bhabdonema nigrovenosa. This is explained by 0. Hertwig as due to the 

 mutual influence of protoplasm and nucleus. Various observers have 

 noticed the change of position associated with the extrusion of polar 

 globules. Herr Schultze observed this in the ova of Amphibians, and 

 especially of Siredon. He explains it in terms of Hertwig's law, that 

 the stretching of the nucleus must take place in the direction of the 

 greatest aggregation of protoplasm within the cell. In mammals a 

 similar turning of the directive spindle seems to occur, as the author's 

 observations on the ova of guinea-pigs, taken along with what Flemming 

 has described, certainly suggest. 



Chemistry of the Nucleus.§ — Prof. A. Kossel points out in connec- 

 tion with adenin that the most recent researches on the importance of the 

 nucleus to the life of the cell (especially the knowledge that when uni- 

 cellular organisms are artificially cut into pieces, only those parts exhibit 

 a complete regeneration which contain a portion of the nucleus), and 

 the importance of the nucleus in impregnation have given an increased 

 importance to the chemistry of the nucleus. 



Among the chemical substances which compose the nucleus, adenin, 

 which has recently been discovered by the author, appears to possess a 

 special importance, since, on account of its composition, C 5 H 5 N 5 , it 

 belongs to the cyanic group of bodies. This substance was obtained 



* This section is limited to papers relating to Cells and Fibres, 

 t Zool. Anzeig., xi. (1888) pp. 190-6. 

 % SB. Phys.-Med. Gesell. Wiirzburg, 1887, pp. 4-5. 



§ Nature, xxxvii. (1887) p. 168, from Proceedings of Berlin Physiological 

 Society, 1887, Nov. 18. 



