ALLEN: EMBRYOLOGICAL TECHNIQUE. 105 



the groove with a moist cloth or camel's hair brush. It should 

 then be thoroughly dried in like manner. All of this requires 

 care and patience, and the permanency of the mount depends 

 upon the neatness with which it is done. 



Then some cement, such as gold size or asphaltum, is run 

 into the groove along all four sides, and the preparation is set 

 aside for the cement to harden. From day to day additional 

 cement is added, until by its successful solidification the groove 

 is completely filled up. The preparation should then be per- 

 manent, with the precaution that it should not be subjected to 

 heat such as might be caused by placing it where direct sun- 

 light will fall upon it. The gelatine can be hardened by the 

 simple process of previously infiltrating the objects to be 

 mounted with a solution of formalin. After mounting, this 

 will gradually diffuse out into the gelatine and harden it. 



This method can be of quite wide application. It could be 

 used in free-hand sections, for instance, one can cut in half the 

 blastulse and gastrulse of the frog. It has always been a prob- 

 lem to properly preserve the halves of such gastrulse that one 

 obtains by cutting the egg in two with a safety-razor blade. 

 Preparations showing various stages of gastrulation were 

 mounted in the manner described, and proved very instructive. 

 Embryological or other objects might well be cut into thick 

 opaque free-hand sections. These should prove extremely in- 

 structive, opening up new and very important fields of em- 

 bryological work. 



Various modifications of the foregoing method may be used. 

 It is quite possible to use hollow-ground slides designed for 

 use with a hanging drop. In such cases as flooding the cell 

 at the completion of mounting, one can use a circular cover 

 slip to cover it, and this can be ringed on a turntable in the 

 usual manner. One must exercise great precaution to avoid 

 overheating the gelatine. When a needle is used to heat a 

 place for the insertion of one of these objects, one should give 

 a little time to allow the gelatine to almost solidify, the object 

 can then be placed in position and blown upon, or the slide 

 placed upon ice so that solidification may be greatly hastened. 

 In this way it is often possible to imbed rather delicate ob- 

 jects with very large cavities, such as the medullary fold stages 

 of Ambystoma. 



