CHAPTER VI 





THE DISSECTION OF PIG EMBRYOS FOR STUDY: DEVELOPMENT OF 

 FACE, PALATE, TONGUE, SALIVARY GLANDS AND TEETH 



As the average student will not have time to study series of embryos sec- 

 tioned in different planes, dissections may be used for showing the form and re- 

 lations of the organs. Cleared embryos mounted whole are instructive, but 

 show the structures superimposed and are apt to confuse the student. Pig em- 

 bryos 10 mm. or more in length may be easily dissected, mounted as opaque 

 objects and used for several years. Success in dissecting such small embryos 

 depends (i) on the fixation and hardening of the material employed; (2) on start- 

 ing the dissection with a clean cut in the right plane; (3) on a knowledge of the 

 anatomy of the parts to be dissected. 



Fixation and Hardening of Material. Embryos fixed in Zenker's fluid have 

 given the best results. They should then be so hardened in 95 per cent, alcohol 

 that the more diffuse mesenchyma will readily separate from the surfaces of the 

 various organs, yet the organs must not be so brittle that they will crumble and 

 break. Embryos well hardened and then kept for two weeks in 80 per cent, 

 alcohol usually dissect well. Old material is usually too brittle, that just fixed 

 and hardened may prove too soft. As a test, determine whether the mesenchyma 

 separates readily from the cervical ganglia and their roots. 



Dissecting Instruments include a binocular dissecting microscope, a sharp 

 safety razor blade, large curved blunt-pointed dissecting needles, pairs of small 

 sharp-pointed forceps and straight dissecting needles small and large. 



Methods of Dissection. In general, it is best to begin the dissection with 

 a clean, smooth cut made by a single stroke with the safety razor blade, which 

 should be flooded with 80 per cent, alcohol. The section is made free hand 

 holding the embryo, protected by a fold of absorbent cotton, between the thumb 

 and index finger. Having made preliminary cuts in this way, the embryo may 

 be affixed with thin celloidin to a cover glass and immersed in a watch-glass con- 

 taining alcohol. We prefer not to affix the embryo, as the celloidin used for this 

 purpose may interfere with the dissection. Instead, a cut is made parallel to 

 the plane of the dissection so that the embryo, resting in the watch-glass upon 

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