COLLODION AND OTHER IMBEDDING MKTHODS. 123 



FISH (Journ. Applied Microscop,, ii, 4, 1899, p. 323) first infiltrates 

 with acetone (which he says may be used as a fixing and dehydrating 

 agent at the same time), then with a 4 per cent, solution of pyroxylin (gun 

 cotton) in acetone ; and, lastly, in an 8 per cent, acetone solution of the 

 same. 



The objects ought to remain in the first bath until very 

 thoroughly penetrated ; days, even for small objects, 

 weeks or months for large ones (human embryos of from 

 six to twelve weeks, for instance). If the object contain 

 cavities, these should be opened to ensure their being filled 

 by the mass. 



When the object is duly penetrated by the thin solution, 

 or solutions if more than one have been employed, it should 

 be brought into the thickest one. This may be done (as first 

 described in this work, 1st edit., 1885, p. 194) by allowing 

 the thin solution to concentrate slowly (the stopper of the 

 containing vessel being raised, for instance by means of a 

 piece of paper placed under it), and making up the loss from 

 evaporation with thick solution. 



APATHY (Mikrotechnili , p. 121) holds, however, that it is 

 preferable to transfer to fresh thick solution, as he finds 

 that a better consistency after hardening is thus obtained. 



154. Imbedding. If the object is such that it can be fixed, 

 by gumming or otherwise, to the holder of the microtome 

 without the intervention of any specially shaped mass of 

 collodion around it, and if the presence of such a mass be 

 not required in the interest of the orientation of the object, 

 or of the production of continuous series of sections, or of 

 very thin sections, no special imbedding is necessary, and as 

 soon as the objects are duly penetrated by the thick solution 

 you may proceed to the hardening part of the process. In 

 other words, it is waste of time to get the object into a 

 special block of collodion if that is not rendered desirable 

 for the reasons above mentioned. But for fine and regular 

 work I hold that it is necessary. 



In that case the objects must at this stage, if it has not 

 been done before, be imhedded that is, arranged in position 

 in the thick collodion in the receptacle in which they are to 

 be hardened. For the usual manipulations see i2(J. 

 recommend the paper thimbles or cylindrical trays, Fig. 2, 



