ZOOLOGY AND BOTANY^ MIOKOSCOPYj ETC. 217 



one of the most far-reactiing of all the adjustments in nature; an 

 adaptation by means of which each organism remains unchanged so 

 long as no change is needed, while it begins to vary whenever varia- 

 tion and race modification are called for. 



Crossing is more advantageous under an unfavourable than under 

 a favourable environment, because it gives variability, and variation 

 is not advantageous when everything else is favourable, while it is 

 advantageous when other things are unfavourable. 



Cell-division.* — C. Eabl's long essay on this subject is divided 

 into two portions ; the first part deals with the cell in repose and 

 during division, while the second will treat of some important 

 histological problems. The author puts in the forefront an account 

 of his methods ; he has made use of the compound of chromic, 

 osmic, and acetic acids, which has been so strongly recommended 

 by Flemming, as well as of chromic acid simply, picric acid, and 

 1/3-1/2 per cent, solutions of chloride of gold; Eetzius' formic 

 acid has also been used. The objection to the use of Flemming's 

 mixture is that the preparations easily darken ; to chloride of gold, 

 that in the summer, even when light is excluded, reduction takes 

 place and the cell-substance is coloured violet ; picric and formic have 

 no advantages over chromic acid. The best results were obtained by 

 a mixture of chromic and formic acids, and by a solution of platinum 

 chloride. The former is prepared by adding to 200 grammes of a 

 1/3 per cent, solution of chromic acid four or five drops of concentrated 

 formic. The fluid must be freshly prepared before using. The 

 objects are cut into small pieces, and after 12-21 hours well 

 washed in water, then slowly hardened in alcohol, being placed first 

 in alcohol of 60-70 per cent., and only after 24-86 hours being 

 placed in absolute alcohol. Platinum chloride has the same advan- 

 tages as chloride of gold, and has not the disadvantage of being 

 reduced. In the chromo-formic solution the chromatin filaments 

 swell up somewhat, so that the longitudinal cleavage of the filaments 

 of the coil and the first stage of the mother-star are generally 

 delayed. In platinum chloride the filaments shrink somewhat, and 

 longitudinal cleavage is effected with extraordinary rapidity. It is 

 clear, then, that the two methods compensate one another. 



For the purposes of staining very intense blue is to be avoided, 

 as the filaments and loops cannot be followed along the whole of 

 their course ; Grenacher's haematoxylin and saflfranin have been almost 

 exclusively used by Eabl. The haematoxylin must have stood for at 

 least two months, and the solution must be very dilute. The best 

 method is to use the haematoxylin and saffranin successively, for if 

 they are mixed a precipitate is formed. Zeiss's 1/18 homogeneous 

 immersion and Abbe's illuminating apparatus were used in making 

 the observations ; green light was preferred ; the slide used is 

 explained with the aid of a diagram ; the figures were drawn with the 

 aid of a Nachet camera, and the drawings are stated to correspond 

 exactly to the preparations ; diagrammatic effects have been carefully 



* Morphol. Jahib., x. (1884) pp. 214-330 (6 pis,). 



