250 XIX. STRUCTURE OF THE THALLOPHYTA. 



Mounting. If, after our study of the preparations is completed, 

 we desire to make permanent preparations from them, we can 

 select for a mountant either glycerine or glycerine-jelly. For 

 carmine preparations 1 per cent, acetic acid may with advantage 

 be added to the mounting medium. The preparations in question 

 must not be transferred direct to the mounting medium, since 

 the cells, as the result of sudden withdrawal of water, would 

 collapse. They are, therefore, first laid in very dilute glycerine, 

 which, by standing exposed to air, very slowly concentrates. 

 The threads can then, without prejudicial results, be transferred 

 to glycerine, or to glycerine- jelly. The glycerine preparations are 

 closed with Canada balsam. The glycerine- jelly preparations needr 

 for a while at least, no further enclosing. 



We will now submit the various fixed and stained preparations 

 to close study, and find that the chromic acid, or chromic acid 

 mixture preparations, stained with forms of carmine, on the one 

 hand, and preparations which are suitably fixed and stained with 

 alum-haematoxylin and ammonia-haematin on the other hand, 

 prove to be the best. It must, however, be explicitly stated that 

 this result refers only to the objects in question, and for other 

 objects some other method, which here is less advantageous, 

 might have the preference. It also happens, only too frequently, 

 that a stain formerly approved fails for unknown reasons, and a 

 conclusion should therefore never be based upon an isolated case. 

 Fixing and staining of the cell-contents has indeed become a 

 special art, which must be learnt, and requires much practice, so 

 that in our first attempts we must be prepared for failures. We 

 have chosen Cladophora as a suitable object for introduction to 

 the various methods of hardening and staining ; whoever washes 

 to limit himself to the surest and mo'st reliable method, will 

 harden in the above way in 1 per cent, chromic acid, and after- 

 wards stain, one portion with either of the recommended carmines, 

 another with alum-haematoxylin. The carmine stains almost 

 always succeed. 



In the carmine preparation (Fig. 95) the nuclei stand out 

 quite sharply. The pyrenoids, together with the rest of the 

 protoplasm, remain almost unstained, and the starch-grains also 

 take no colour. The nuclei, to which we specially turn our 

 attention, are distributed pretty uniformly in the cell ; they lie 

 on the inner side of the chlorophyll layer, and project into the 

 cavity of the cell. Each nucleus shows a more darkly-stained 



