HEMATOXYLIN-STAINED, 48-HOUR CHICK 91 



course, on the lower surface of the embryo. Having maneuvered the embryo 

 in the saline solution in the watch glass until it is in the upside-down position 

 required, the water should be drained off with the aid of a pipette, which is 

 run rapidly with a circular motion around the outside of the blastoderm while 

 the water is drawn up. As experience will soon show, any attempt to drain 

 the water up a stationary pipette will result in the embryo being drawn out in 

 the direction in which the water is being sucked. A little practice in running 

 the pipette round and round the outside of the blastoderm and about a milli- 

 meter away from it will enable the operator to strand the embryo perfectly 

 stretched in all directions. Under no circumstances whatever should a needle 

 be used in an endeavor to arrange the embryo because the point will adhere 

 to the blastoderm from which it cannot be detached without damage. If the 

 embryo is not flattened and spread out satisfactorily, it is necessary only to 

 add a little clean saline solution with a pipette and repeat the operation. 



A piece of coarse filter paper or paper toweling is cut into a rectangle of 

 such size that it will drop easily into a Syracuse watch glass. An oval or cir- 

 cular hole is cut in the middle of this (done most easily by bending it in two 

 and cutting a semicircle) of such a size as will cover exactly those areas of the 

 embryo which are to be retained. That is, if the embryo alone is required, the 

 hole may be relatively small, while if it is desired to retain all the area vas- 

 culosa with its sinus terminalis, the hole must be correspondingly enlarged. 

 The hole must not be larger, however, than the blastoderm removed from the 

 egg because the next operation is to cause the unwanted extraembryonic 

 regions to adhere to the paper, leaving the embryo clear in the center. By this 

 means alone will the embryo be prevented from contracting and distorting 

 when fixative is applied to it. Such data as are pertinent may be written on 

 the edge of the paper rectangle in pencil. Then the paper is dipped in clean 

 saline solution. If the saline used has already become contaminated with egg 

 white, a sharp puff of air should be directed at the hole to make quite certain 

 that a film of moisture does not extend across it since the bubbles so produced 

 always disrupt the embryo if this film is left. Now the rectangle of filter paper 

 is dropped on top of the stretched embryo in such a manner that the embryo 

 does not become distorted. This is, in point of fact, a great deal easier than it 

 sounds although a few false trials may be made by the beginner. The author's 

 procedure is to place one end of the rectangle on the edge of the watch glass 

 nearest him, taking care that it does not touch the blastoderm, and then to let 

 the paper down sharply. The edges of the blastoderm must be in contact with 

 at least two thirds of the periphery of the hole if it is to remain stretched. As 

 soon as the paper has been let down, the end of a pipette or a needle should 



