504 



HISTOLOGY 



This is the best way known in most cases of seeing an accurate picture 

 of the details of structure in a tissue. Good results can be attained by 

 staining the tissue after fixation, and before it is dehydrated. In that 

 case, when the sections are cut and mounted on the slide, they may be at 

 once freed from paraffin and mounted in the balsam. This way of 

 staining in toto or in bulk, although shorter, seldom shows the detail and 

 accuracy of sections stained on the slide. It is a valuable method, never- 

 theless. 



In case embedding in celloidin is decided upon, the material is dehy- 

 drated and immersed in a solution of celloidin or photoxylin dissolved in 

 ether or absolute alcohol, or, as is more commonly done, in a mixture 

 of both. After a long soaking to allow of a good infiltration, the solution 

 is thickened by evaporation or the addition of a stronger solution, when 

 the mass, now nearly solid, may be hardened in chloroform or its vapor, 

 and then cut into sections which are not as thin as paraffin sections, but 

 usually have a peculiar beauty of their own, due to their not having been 

 subjected to the shrinking action of cooling paraffin. As in the paraf- 

 fin method, these sections can be stained separately before mounting, or 

 the material can be previously stained in bulk, which is perhaps better 

 for the celloidin method, and permits of immediate mounting when cut. 

 Otherwise the sections are stained as "free sections" or, in rare cases, 

 on the slide. 



The student should consult Lee for the cutting of sections of bone, 

 etc., and other exceptional cases. 



FIRST EXAMPLE 

 Secure a salamander (Necturus preferred). 



