92 SPECIFIC EXAMPLES OF SLIDE-MAKING 



be used to press lightly on the edges of the paper, where it is in contact with 

 the blastoderm, to make sure that it will adhere. 



The embryo is now ready for fixing. The choice of a fixative, naturally, 

 must be left to the discretion of the operator. The author's preference, where 

 hematoxylin is to be employed for staining, is for a mercuric mixture, such as 

 the solution of Gilson (see p. 10). If much embryonic work is to be under- 

 taken, reference should be made to Gray's Microtechnique for the formula of 

 Gerhardt's fixative, which is better. The disadvantage of the customarily-used, 

 picric acid formulae is that they interfere seriously with subsequent staining by 

 hematoxylin. The fixative should be applied from an eye-dropper type pipette 

 in the following manner. First, a few drops are placed on the center of embryo, 

 so that a thin film of fixative is spread over it. After a moment or two a little 

 more may be added with a circular motion on the paper which surrounds the 

 embryo. Again the paper should be pressed on the periphery of the blastoderm 

 with a needle or the end of the pipette to make sure that the adherence is 

 perfect, and the whole should be left for a moment or two before being shaken 

 gently from side to side to make quite certain that the embryo is not sticking 

 to the watch glass. If it is sticking, the end of the pipette containing the fixa- 

 tive should be slid under the edge of the paper, and a very gentle jet of fixative 

 used to free the embryo. As soon as the embryo is floating freely in fixative, 

 the Syracuse watch glass may be filled up with fixative and placed to one side, 

 while the same cycle of events is repeated with the next embryo. After about 

 10 minutes in the fixative, the paper may be picked up by one corner and 

 moved from reagent to reagent without the slightest risk of the embryo 

 becoming either detached or damaged. Care, of course, must be taken not to 

 pick up the paper with metal forceps unless the instrument has been waxed 

 first because the mercuric chloride in the fixative will damage the metal. It is 

 the author's custom to leave the embryos in the watch glasses for about 30 

 minutes before picking them out and transferring them to a large jar of the 

 fixative, preferably kept in a dark cupboard. The total time of fixation is not 

 important but should be not less than one day nor more than one week. 

 When the embryos are removed from the fixative, they should be washed in 

 running water overnight and may then be stored in 70 per cent alcohol for an 

 indefinite period. 



When one is ready to stain a batch of embryos, it is necessary only to take 

 them from 70 per cent alcohol back to distilled water until they are thoroughly 

 rehydrated, and then transfer them to a reasonably large volume of Carazzi's 

 hematoxylin where they may remain overnight. The gravest mistake which 

 can be made in this type of staining is to stain initially for too short a period. 



