CH. X] WAX MODELS 325 



Further the difficulty of getting a true conception of the object by study- 

 ing only two dimensions in the sections is very great, hence a model giving 

 all three dimensions becomes almost a necessity for the beginner in embry- 

 ology, and is of enormous advantage to an investigator in working out the 

 true form and relation of complex structures. 



The principles involved in the construction of a model are exceedingly 

 simple : — 



1. It is necessary that the embryo or other object to be modeled should 

 be cut into a series of sections of definite thickness. 



2. The sheets of modeling material must be as much thicker than the 

 sections as the model is to be larger than the original. 



3. The sections must be drawn as much larger than the actual specimen 

 as the model is to be larger than the object. 



4. The drawings with the desired outlines must be made directly upon 

 or transferred to the sheets of modeling material which are then cut out, fol- 

 lowing the lines of the drawing. 



5. The different plates of modeling material representing all the sections 

 are then piled up, in order, thus giving an enlarged model of the object with 

 all its parts in proper position and in true proportions. 



MODELS OF WAX 



\ 490. Wax Models. — For making wax models, bees-wax 820 grams, 

 paraffin 270 grams, and resin 25 grams, are melted together and thoroughly 

 mixed. 



To get the sheets of wax of the proper thickness two methods are 

 available : — 



( 1 ) The hot wax is poured into a vessel containing hot water. The wax 

 spreads out into an even layer over the hot water and is allowed to cool. 

 While it is solidifying it should be cut free from the edges of the vessel. Of 

 course by calculation and experiment one can put in the right amount of wax 

 to get a plate of a given thickness. 



(2) One must have a wax-plate machine consisting of a flat surface — 

 planed cast iron is good — with some means of obtaining raised edges. If these 

 are adjustable by a micrometer screw it is simple to set them properly for the 

 desired thickness of plate. Then there must be a hot roller. The hot wax is 

 poured on the plate and with the hot roller resting on the raised edges, the 

 wax is rolled out into a plate. It cools quickly and may be removed for 

 another plate. This is the most rapid and satisfactory method of prepar- 

 ing the plates. By using a brush with turpentine the paper with the drawing 

 can be wet and then with the hot roller cemented to the plate before that has 

 been removed from the machine. 



The wax plate is cut with a sharp instrument, following the outlines of 

 the object which has been traced upon it by the aid of a camera lucida or the 

 projection microscope. The sections are piled together, some line or lines 

 obtained from a drawing or photograph of the specimen before it was imbedded 



