CHAPTER VI 



THE DISSECTION OF PIG EMBRYOS: DEVELOPMENT OF FACE, 

 PALATE, TONGUE, SALIVARY GLANDS AND TEETH 



THE DISSECTION OF PIG EMBRYOS 



As the average student will not have time to study series of embryos sectioned in differ- 

 ent planes, dissections may be used for showing the form and relations of the organs. Cleared 

 embryos mounted whole are instructive, but show the structures superimposed and are apt to 

 confuse the student.- Pig embryos 10 mm. or rnore in length may be easily dissected, mounted 

 as opaque objects, and used for several years. Success in dissecting such small embryos de- 

 pends: (1) on the fixation and hardening of the material employed; (2) on starting the dissec 

 tion 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 

 difl^use mesenchyma will readily separate from the surfaces of the various organs, yet the 

 organs must not be so brittle that they wiU 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. 



DissectLQg instruments include a binocular disgecting microscope, a sharp safety 

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

 ceps, and straight dissecting needles small and largfe. 



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 im- 

 mersed in a watch glass containing 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 this flat surface, 

 will be in a fairly stable position. It may thus be held in any convenient position by resting 

 the convex surface of a curved blunt dissecting needle upon some part not easily injured. 

 The dissection is then carried on under the binocular microscope, using the fine pointed for- 

 ceps, dissecting needles, and a small pipette to wash away fragments of tissue. 



Whole Embryos. — For the study of the exterior, whole embryos may be affixed with 

 celloidin to the bottoms of watch glasses which may be stacked in widfe-mouthed jars of 80 

 per cent, alcohol. The specimens may thus be used several years at a saving of both time and 

 material. Preliminary treatment consists in immersion in 95 per cent, alcohol one hour, in 

 ether and absolute alcohol at least thirty minutes, in thin celloidin one hour or more. Pour 

 enough thin celloidin into a Syracuse watch glass to cover its bottom, and immerse in this a 

 circle of black mat paper, first wet with ether and absolute alcohol. Pour off any surplus cel- 

 loidin, mount embryo in desired position and immerse watch glass in 80 per cent, alcohol, 



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