CHAPTER VI 



THE DISSECTION OF PIG EMBRYOS : DEVELOPMENT OF THE 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 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 starting the dissection with a dean 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 pre- 

 liminary cuts in this way, the embryo may be affixed with thin celloidin to a cover glass and 

 immersed 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 forceps, 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 wide-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 



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