216 CHESTER H. HEUSER 



Casts of the ventricles, in embryos measuring from 12 to 22 

 mm., were made after the following method. Longitudinal 

 sections of the entire embryo, made slightly to the right side of 

 the median plane, were prepared as above described. Then, 

 under the binocular microscope, obstructing portions of the brain 

 walls were dissected off, and the ventricles were cleaned of all 

 precipitation with a small brush and syringe. Next the embryo 

 was taken from the alcohol and pinned down to a piece of cork 

 and with narrow pointed strips of filter paper all the alcohol was 

 removed from the cavities of the brain. In a large measure the 

 success of the cast depends upon the completeness of the removal 

 of the alcohol. The cast was made by filling the cavities with 

 black wax with the aid of a fine electrically heated needle. A 

 small piece of wax was carried with a cold needle to the proper 

 place and then melted with the electric needle. This process 

 was repeated until the ventricles had been filled. Then if desired, 

 white wax was melted over the upper surface of the dissection 

 for holding the parts more closely together. Moreover, the 

 white background thus provided contrasts sharply with the black 

 cast. Lastly the preparation was turned upside down and the 

 brain wall was carefully dissected away from the wax. For 

 making a permanent preparation of the cast, it is best to mount it 

 in a dry cell on a microscopic slide. Casts thus made show a 

 little more than one-half the ventricles. They can be studied 

 with the binocular microscope, and drawings can be made quite 

 as satisfactorily as from larger models. After embryos exceed 

 20 mm. in length the interventricular foramen is so small that it 

 is difficult to get the lateral ventricles completely filled, and after 

 24 mm. the lateral recesses can hardly be cast. 



For the electric needle a piece of no. 18 copper wire is coated, except 

 for about 15 mm. at one end, with a thin layer of some insulating paste 

 which will harden and be resistant to heat. The commercial 'caemen- 

 tium' is very satisfactory. The bare end is ground down to a fine point 

 which is the working part of the instrument. Behind this, covering a 

 length of about 20 mm., a piece of no. 32 German silver wire, about 20 

 cm. long, is wrapped around the insulated copper wire, the coils being 

 well insulated from each other and covered with caementium. Attached 

 to the ends of the German silver wire are small copper wires which run 



