OP ARTS AND SCIENCES. 29 



before with the same volume of platinum cliloriclc (one fourth per 

 cent), everything may be well preserved and hardened except the 

 yolk. But this fluid cannot be used with success unless the egg has 

 been first killed by another agent; for eggs placed in this fluid continue 

 to live for a considerable time, and may even pass through one or 

 two stages of cleavage. It is therefoi'e necessary to use some reagent 

 that kills instantly. For this purpose a weak solution of osmic acid 

 may be used. 



The eggs are placed in a watch-glass with a few drops of sea-water, 

 and then a quantity of osmic acid (one half per cent) equal to that of 

 the sea-water is added. After five to ten minutes the eggs are trans- 

 ferred to the mixture of chromic acid and platinum chloride, and left 

 for twenty-four hours or more. This fluid not only arrests the pro- 

 cess of blackening, but actually bleaches the egg to a considerable 

 extent. After this treatment it is an easy matter to separate the 

 blastoderm from the yolk by needles ; and the preparations thus ob- 

 tained may be stained at once, and then treated with alcohol and 

 mounted in balsam. 



The value of such preparations must be measured by the accuracy 

 and clearness with which they present the conditions in the living 

 state. A careful study of the preparations shows that the method 

 can be relied on in every particular. The osmic acid fixes the living 

 conditions more perfectly than any other reagent at present known, and 

 the chrom-platinum mixture completes the work of hardening without 

 shrinkage or swelling. The finest details of the cleavage lines, the 

 cleavage cavity, and the nuclear figures, are well preserved. The 

 relation of the blastoderm to the protoplasmic mantle enveloping the 

 vitellus, and all the particulars in regard to the origin of the so-called 

 " free nuclei," are satisfactorily shown. No violence is required in 

 order to free the blastoderm from the yolk, as a clean separation is 

 usually eflTected by the action of the acids. In this separation the 

 protoplasmic mantle invariably goes with the blastoderm, in the older 

 as well as in the younger stages of development. 



3. For sectioning, the embryonic portions of the egg need not be 

 separated from the yolk. But before transferring the eggs from the 

 chrom-platinum solution to the different grades of alcohol (fifty to one 

 hundred per cent), the egg-membrane should be broken or perforated 

 by the aid of needles on the side opposite the blastoderm, in order 

 that the alcohol may reach the egg readily, as otherwise the mem- 

 brane wrinkles badly, and often injures the embryonic portion. For 

 the embryonic stages, the above method of hardening has not been 



