The following method may be used for _staining embryos_: Dilute Delafield’s hematoxylin with four times its volume of water. To every 6 cubic centimeters of this diluted hematoxylin, add one drop of Kleinenberg’s undiluted picrosulphuric acid, and leave specimen in the fluid thus prepared until it is stained through. This will require from one to three hours. Now pass up through the series of alcohols to 70 per cent. Next extract the excessive stain with 1 per cent. hydrochloric acid in 70 per cent. alcohol. Wash repeatedly with 70 per cent. alcohol to free from the acid; then transfer the specimen to 80 per cent. alcohol and leave in this for several hours for complete removal of the acid; then transfer to 95 per cent. alcohol for thirty minutes. Allow the specimen to remain in absolute alcohol for one hour. Introduce a layer of oil of cloves or xylol beneath the alcohol. This may be done by gradually allowing the fluid to run down the side of the bottle. After the embryos have sunk into the oil and begun to appear transparent, remove the fluid and add fresh oil. After the specimen is sufficiently transparent, mount in balsam, supporting the cover slip so that it will not rest on the embryo. In staining for section, place the embryo in borax carmine from the 50 per cent. alcohol and leave twelve hours. Then wash in 50 per cent. alcohol; after which transfer to 70 per cent. alcohol for six hours. Clarify in oil of cedar or oil of cloves, and place in melted paraffin for two hours. Imbed in paraffin, and section. Embryos may be hardened, imbedded, and sectioned after the usual methods, using either paraffin or celloidin, and the sections stained with hematoxylin and eosin after sectioning. POINTS TO BE OBSERVED IN THE STUDY 1. THE EMBRYO TWENTY-NINE TO THIRTY-FOUR HOURS OLD WITH FROM TEN TO FOURTEEN SOMITES. A study of the egg. In opening the egg, observe that the shell membranes are double. Observe that the shell is porous. Note the air cell at the large end of the egg and note that the space or cell lies between the outer and the inner shell membrane. After the egg contents have dropped into the salt solution, note extending from the ends of the yolk the twisted denser cords of albumen. These are the chalazæ, which act as stays to the yolk. Note that the yolk is surrounded by a delicate membrane. This is the vitelline membrane. The yolk is the true ovum and serves as food for the developing embryo. A study of the living embryo. Note the amount of yolk that is covered by the blastoderm. Note the slipper-shaped, transparent center of the blastoderm. This is the area pellucida. In the center of this there is a narrow white streak. The area opaca is the area lying external to the area pellucida. In this there is the area vasculosa. A study of the embryo entire, including the vascular area.