194 COLLODION AND OTHER IMBEDDING METHODS. 



proportion of alcohol. The tablets cost three marks (= three 

 shillings) each. A single tablet would, I think, suffice for 

 imbedding many hundreds of embryos. 



There is a strife of opinion amongst authorities as to the 

 relative merits of celloidin and common collodion. DUVAL, in 

 a restatement of the method (Journ. de Micr., 1888, p. 197), 

 teaches that celloidin has no real advantage over common 

 collodion, whilst the latter has the advantage of being more 

 transparent. SCHIEPFEEDECKER, also restating the method, 

 declares that celloidin has " many points of superiority " 

 (Zeit. f. wiss. Mik., v, 4, 1888, p. 504). On carefully reading 

 Schiefferdecker's paper, however, it appears that the only 

 reason alleged in support of this superiority is that it is more 

 " convenient " to make a thick mass by making a strong 

 solution of celloidin than by allowing common collodion to 

 concentrate by evaporation. 



Personally I incline to Duval's point of view. The supe- 

 riority of celloidin, if it exists, is mainly of the nature of a 

 mere matter of convenience. Otherwise there is but little to 

 choose between the two, and therefore in this work the terms 

 collodion and celloidin are used indifferently. 



Photoxylin (KRYSINSKY, VIRCHOW'S Archiv, cviii, 1887, 

 p. 217 ; BUSSE, Zeit. f. wiss. Mik., ix, 1, 1892, p. 47) is a dry 

 substance, of the aspect of cotton wool, and chemically nearly 

 related to celloidin. It can be obtained either from SOBERING 

 or G-RUBLER. It gives a clear solution in a mixture of equal 

 parts of ether and absolute alcohol, and should be used in 

 exactly the same way as celloidin. It has the very great 

 advantage of affording a mass which after hardening in 85 

 per cent, alcohol remains perfectly transparent. 



298. Preparation of Objects. The objects must first be 

 thoroughly dehydrated with absolute alcohol. They are then 

 soaked till thoroughly penetrated in ether, or, which is better, 

 in a mixture of ether and absolute alcohol. DUVAL (1. c.) 

 takes for this purpose a mixture of ten parts of ether to one 

 of alcohol ; SCHIEFFERDECKER (and the majority of workers) a 

 mixture of equal parts of ether and alcohol ; TUBBY (in Nature, 

 November 17th, 1892, p. 51) advises a mixture of four parts 

 of ether and one of alcohol. But the point is one of no 

 great importance. 



