186 Specific Examples of Slide Making 



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 

 circular 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 vasculosa 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, leav- 

 ing 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. The paper is then 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. The rectangle of filter paper 

 is now 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 be used to press 

 lightly on the edges of the paper where it is in contact with the blastoderm, 

 to make sure that it will adhere. 



The embryo is now ready for fixing. The choice of a fixative, naturally, 

 must be left to the discretion of the operator. The author's preference, 

 where hematoxylin is to be employed for staining, is for a mercuric mix- 

 ture, such as the solution of Gilson. If much embryonic work is to be 

 undertaken, reference should be made to Gray's "Microtomist's Formu- 

 lary and Guide" for the formula of Gerhardt's fixative, which is better. 

 The disadvantage of the customarily used, picric acid formulas is that they 

 interfere seriously with subsequent staining by hematoxylin. The fixative 



