THE COLLODION BATH. 159 



cretion. To prepare a medicinal collodion according to the 

 Prussian Pharmacopoeia, you take for each tablet 720 grains 

 of ether and no alcohol, as the celloidin already contains the 

 prescribed 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, 

 re-stating the method lately (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. SCHIEFFERDECKER, also re-stating 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 solu- 

 tion of celloidin than by allowing common collodion to con- 

 centrate by evaporation. 



Personally I incline to DuvaPs point of view. The supe- 

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

 matter of convenience. (Unless, indeed, the mass recom- 

 mended by Apathy, 292, of which the description reaches 

 me whilst preparing these sheets for the press, should prove 

 to be really superior.) Otherwise there is hardly a pin to 

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

 collodion and celloidin are used indifferently. 



291. 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 (I. 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. 



This stage may be omitted if the objects are of a sufficiently 

 permeable nature, and they may be brought direct from 

 alcohol into the collodion bath. 



292. The Collodion Bath. The next step is to get the objects 

 ponutnited with thick collodion. The secret of success here 



