Method XVIII gg 



It will be seen that this mixture consists of weak alcohol to 

 which has been added lo per cent, of formaldehyde and about 

 2 per cent, of acetic acid. The alcohol tends to counteract the 

 swelling effects of the other two ingredients. The mixture pene- 

 trates and fixes well. Material may remain in it indefinitely, 

 but it is thought better to transfer it to 70 per cent, alcohol after 

 24-48 hours. It needs no special preliminary washing. 



D. 10 per cent, formalin (see X, A, p. 63). 



All of the above fluids keep well, and may be made up in 

 quantity and kept in stock. Bottles containing them should 

 have tightly fitting stoppers to prevent dilution by evapora- 

 tion. It is recommended that either the fluids themselves or 

 the reagents for making them be kept on hand, so that, when a 

 specimen is obtained, it may be subjected to the fixing fluid 

 with as little delay as possible. 



It is also advised that wide-mouthed bottles of various sizes 

 be kept on hand. The ordinary 3 oz. quinine bottle will be 

 found excellent for the smaller embryos, and fruit jars of J^ 

 pint, I pint, and i quart sizes will do for the larger foetuses. 



REMOVAL AND FIXATION. 



In case of the smaller embryos (3— 5 mm. in length), carefully 

 open the chorion before placing them in the fixing fluid. When 

 the embryo is located, a portion of the chorion may be removed 

 and openings made in the amnion so that the fluid may directly 

 come in contact with the specimen. 



If there is reason to believe the embryo is less than 2 mm. 

 in length, it is necessary to proceed with much caution. Exam- 

 ine the specimen under a lens, either in its own fluid or in a 

 watch-glass of physiological salt solution (VII, A, p. 43), and, 

 if there is reason to believe that the stage of the infolding of 

 the blastoderm is not yet complete, place the whole specimen 

 directly in the fixing fluid. In this case the specimen will be 

 quite small. 



If, on the other hand, the chorion with its villi seem to sur- 

 round the specimen apparently forming a ciliated sphere, and 



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