^6 THE DISSECTION OF PIG EMBRYOS FOR STUDY 



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 con- 

 sists 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 celloidin, mount embryo in desired position and immerse watch-glass 

 in 80 per cent, alcohol, in which the specimen may be kept indefinitely. Embryos 

 may also be mounted in gelatine-formalin solution in small sealed glass jars. 



Lateral Dissections of the Viscera. — Dissections like those shown in Figs. 

 139 and 140 may easily be prepared in less than an hour, and make valuable 

 demonstration and laboratory specimens. Skill is required to demonstrate most 

 of the cerebral nerves, but the central nervous system, cerebral and spinal ganglia 

 and viscera may easily be exposed. Starting dorsally, make a sagittal section 

 of the embryo slightly to one side of the median line and avoiding the umbilical 

 cord ventrally. With the embryo resting on the flat sectioned surface, begin 

 at the cervical flexure and with fine forceps grasp the ectoderm and dural anlage 

 at its cut edge, separate it from the neural tube and pia mater and strip it off 

 ventralwards exposing the myelencephalon and cervical portion of the cord. 

 As the mesenchyma is pulled away, the ganglia and roots of the cerebral nerves 

 will be exposed. The mesenchyma between the ganglia and along the nerves 

 may be removed with the end of a small blunt needle. Care must be exercised 

 in working over the mesencephalon and telencephalon of the brain not to injure 

 the brain wall, which may be brittle. By starting with a clean dissection dor- 

 sally and gradually working ventrad, the more important organs may be laid 

 bare without injury. The beginner should compare his specimen with the dis- 

 sections figured and also previously study the reconstructions of Thyng (191 1) 

 and Lewis (1902). 



Lateral dissections of embryos 18 mm. and 35 mm. long show infinitely better 

 than sections the form and relations of the organs, their relative growth and 



