ALLEN: EMBRYOLOGICAL TECHNIQUE. 107 



can be made out with great clearness. It is even possible to 

 see the outlines of the cranial and spinal nerve roots. 



Experience in teaching by means of these preparations has 

 demonstrated the ease with which they can be studied and 

 understood. It is quite possible to mount tadpoles with the 

 dorsal surface uppermost. A special method of technique is 

 necessary to accomplish this last named end. Either Canada 

 balsam or dammar is heated in an oven for some days until it 

 will harden immediately upon cooling. Heated balsam or 

 dammar of this character is then dropped upon slides. These 

 drops should be of the proper width, to fill the space beneath 

 a cover slip and to at the same time maintain the desired thick- 

 ness. It is not the least difficult to get drops which may be 3 

 or 4 millimeters in depth. They can be measured so as to give 

 any proportion desired between breadth and depth. This can 

 be done by heating the slide with the adherent drop over an 

 alcohol lamp. It often chances that there are numerous small 

 bubbles in such drops. These can be brought to the surface 

 by placing the slides for some hours in a paraffin oven. They 

 can then be removed by skimming the surface after heating, 

 or in many cases they can be burst by application of a flame. 



Large numbers of such slides can be prepared at a time and 

 saved for subsequent use. They can be used, of course, for 

 various mounts where it is desired to have the cover slip 

 propped some distance above the slide, and in which immedi- 

 ate use is desirable. 



Tadpoles, or other objects are mounted in the drop in the 

 following manner : First, the drop is heated over a flame, being 

 held there a moment or so inverted above it; then with a 

 scalpel, previously dipped in a solvent such as xylol, a groove 

 is made in it. This can be formed to suit the size and shape 

 of the object to be mounted. The object is then placed in it — 

 the sides of the groove pressed into contact with it in such a 

 'way as to hold it in the desired position. Then the space about 

 the object is filled with a drop of soft balsam, and a cover slip, 

 after being previously heated, is pressed down gently but 

 firmly upon the surface of the drop. Care must be taken to 

 prevent the inclusion of bubbles at this stage. A little prac- 

 tice will enable one to do this without difficulty. 



The preparation should be laid aside for a week or more, 

 and then examined. In some instances it will be found that the 



3 — Univ. Sci. Bull., Vol. IX, No. 8. 



