PKACTICAL HINTS 



THE object of this book is not merely to lay before the reader the 

 best ascertained results of embryology, it is also designed to 

 indicate the directions in which further research may be most ad- 

 vantageously prosecuted, and to suggest reliable methods of pursuing 

 such researches. Incidentally defects in the methods employed by 

 some investigators, and the possible bearing of these defects on their 

 results, will be pointed out. 



In the present chapter some general instruction will be given on 

 methods of procedure which are applicable to all, or nearly all classes 

 of embryo, while special methods will be described when each separate 

 phylum is described. 



When one endeavours to work out the life-history of an animal 

 the first step is to observe the larvae or embryos in the living state. 

 In many cases a large number of points can only be made out in the 

 living embryo, since the tissues are then in their natural state of tur- 

 gescence, and living protoplasm is relatively transparent. The next 

 step is to preserve or fix the embryos, dehydrate and clear them and 

 mount them whole. 



Fixing or preservation consists in adding some reagent to the 

 specimen to be preserved which will form a stable and more or less 

 solid compound with the protoplasm of the organism. This compound 

 enables the form of the organism to be retained during the process of 

 dehydration, and the macerating and deforming effects of the diffusion 

 currents produced in this process to be resisted. Dehydration, is 

 the removal of the water by successive immersion of the object in 

 different grades of alcohol ; clearing, is infiltration of the tissues by 

 an oil like oil of cloves, cedar oil, etc., which renders them transparent. 



Now the reagent which forms the strongest compound with 

 protoplasm and preserves in it the nearest resemblance to its living 

 condition is the solution of osmium tetroxide in water, usually 

 erroneously called osmic acid. For effective fixation a solution of at 

 least '25 per cent must be used. "Osmic acid" has two disadvantages, 

 it produces a very black stain which consists of the metal osmium, 

 and it is apt to render the tissues brittle. Further, if applied to objects 



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