638 APPENDIX. 



show the relative position and general shape of the organ as seen from the dorsal or ventral 

 side. If desired, the sketch can be shaded to represent the stomach in perspective. 



Drawings of two or three different structures to show their interrelation can be made 

 in this way, so long as the structures do not become too complicated. Sometimes it is neces- 

 sary to draw only from every third or fourth section. 



Plastic or Wax Reconstructions. — These are simply wax models of an organ or a system, 

 and are built up from pieces of wax which represent magnified sections of the embryonic 

 structures. Here, as in the case of graphic reconstructions, serial sections of a known 

 thickness are necessary. A camera lucida also is necessary. Wax plates for this work can 

 be purchased from firms that trade in scientific apparatus. Plates i mm. or 2 mm. in thick- 

 ness are most convenient, for they must represent a definite magnification of the thickness of 

 the sections from which the reconstruction is to be made. For example, if the magnifica- 

 tion is 50 times and the sections 20 microns thick, the plates should be 50 times 20 microns, 

 i.e., one millimeter in thickness. 



As a simple example of this kind of reconstruction, suppose a model of the embryonic 

 liver is to be made, the sections being 20 microns thick and 50 the chosen magnification. 

 Select the section containing the cephalic edge of the liver and with the camera lucida trace 

 the outline of the organ and the median line of the section on one of the wax plates. The 

 plate should be 1 mm. in thickness because that represents the thickness of the section 

 magnified 50 times. Lay the plate upon a smooth hard surface and, with a thin-bladed 

 knife, cut out (he piece indicated by the outline tracing of the liver. 



Do exactly the same with successive sections, keeping the corresponding pieces of wax 

 in order. Then arrange the pieces of wax, that is pile them up, in the same order, being 

 careful that the lines traced upon them to represent the median lines of the sections come in 

 the same plane. (This plane represents the median sagittal plane of the embryo.) When 

 properly piled, the wax plates form a mass which represents the embryonic liver magnified 

 50 times. The plates are fastened together by passing a warm metal instrument over their 

 edges, thus slightly melting the wax. 



By the same method, whole systems o_ organs or even whole embryos can be reconstructed. 

 With complicated structures, however, the utmost care is required to place the various parts 

 in their proper relative positions. 



In case of structures consisting of many parts, like the vascuiar system, ior example, it is 

 scarcely possible to trace and cut out the various pieces of wax and put them together again 

 without fastening together each time the parts that belong to each section. The best method 

 to follow in such cases is as follows: Make the camera lucida sketch, including the median 

 line, of each section on a sheet of paper. By means of carbon paper transfer the tracings of 

 the various parts to the wax. Cut out the pieces of wax and lay them in their proper posi- 

 tions on the sheet of paper. Then fasten them all together by means of pieces of fine copper 

 wire heated to such a temperature that when laid upon the vax they will sink into it. If this 

 is done with the pieces belonging tc each section, then thu pieces belonging to successive 

 sections can be piled and fastened together by passing a warm metal instrument over their 

 edges. 



References. 



Lee, A. B.: "The Microtomist's Vade-Mecum." Sixth Edition. P. Blakiston's Son 

 & Co. Philadelphia, 1905. 



Mallory, F. B., and Wright, J. H. : "Pathological Technique." Third Edition. Saun- 

 ders & Co. Philadelphia, 1908. 



Minot, C. S.: "A Laboratory Text-book of Embryology." P. Blakiston's Son & 

 Co. Philadelphia, 1903. 



