DS 11.10 



DYE STAINS OF GENERAL APPLICATION 



277 



persuade it to flatten out on the bottom in 

 an upside down position, that is, so that 

 that portion of the embryo which was jne- 

 viously in contact with the yolk is now di- 

 rected toward the operator. To determine 

 which side of the embryo is uppermost 

 requires considerable practice, unless the 

 primary curvature of the head toward the 

 right has already started. The best point 

 of examination is the heart, which lies, of 

 course, on the lower surface of the embryo. 

 Having maneuvered the embryo in the 

 saline in the watch glass until it is upside 

 down, the water should now be drained off 

 with the aid of a pipet which is run rapidly 

 with a circular motion round and round 

 the outside of the blastoderm while the 

 water is being drawn up. As experience 

 will soon show, any attempt to drain the 

 water up a stationery- pipet will result in 

 the embryo being drawn out in the direc- 

 tion in which the water is being sucked. A 

 little practice with the pipet being run 

 round and round the outside the blasto- 

 derm and about a milUmeter away from it 

 ■will enable the operator to strand the em- 

 bryo so that it is perfectly stretched in all 

 directions. Under no circumstances should 

 a needle be used to arrange the embryo, or 

 the point will adhere to the blastoderm 

 from which it cannot be detached without 

 damage. If the embrj-o is not flattened 

 and spread out satisfactorily, it is only 

 necessar}^ to add a little clean saline with a 

 pipet and repeat the operation. 



A piece of coarse filter paper or paper 

 towel is now taken and cut into a rec- 

 tangle of such size that it will fit easily 

 into a Syracuse w^atch glass. An oval or 

 circular hole is then cut in the middle of 

 this (most easily done by bending it in two 

 and cutting from it a semicircle) of such 

 a size as will exactly cover those areas of 

 the embryo which it is desired to retain. 

 That is, if only the embryo is required, the 

 hole may be relatively small; if it is neces- 

 sary to retain the whole of the area 

 vasculosa with its sinus terminale, the hole 

 must be correspondingly enlarged. The 

 hole must not, however, be larger than 

 the blastoderm removed from the egg, be- 

 cause the next stej) causes the unwanted 

 extraembryonic regions to adhere to the 

 paper, leaving the embryo clear in the 



center. By this means alone will the em- 

 bryo be prevented from contracting and 

 distorting when fixative is applied to it. 

 Having prepared a rectangle of a suitable 

 size with a suitable hole in the center, such 

 data as are pertinent may be written on 

 the edge in pencil. The paper is then 

 dipped in clean saline. If the saline used 

 has already become contaminated with 

 egg white, a sharp puff should be directed 

 at the whole to make quite certain that a 

 film of moisture does not extend across it 

 because tlie 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. That is a great deal easier than 

 it sounds, though a few false trials may be 

 made bj- the beginner. The writer's pro- 

 cedure is to place one end of the rectangle 

 on the edge of the watch glass nearest to 

 him, taking care that it does not touch the 

 blastoderm, and then to let it 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 pipet 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 to be fixed, 

 and the choice of a fixative must naturally 

 be left to the discretion of the operator. 

 The author's preference, when hematox- 

 ylin is subsequently to be employed for 

 staining, is for the mercuric-chromic-form- 

 aldehyde mixture of Gerhardt (Chapter 

 18, F 3600.1010 Gerhardt 1901). The dis- 

 advantage of the customarily used picric 

 formulas is that they interfere seriously 

 with subsequent staining by hematoxj-lin. 

 The fixative should be applied from an 

 eye-dropper type pipet in the following 

 manner. First a few drops are placed on 

 the center of the embryo so that a thin 

 film of fixative is spread over it. After a 

 moment or two a little more may be added 

 with a circular motion on the paper which 

 surrounds the embryo. The j)aper sliould 

 again, at this point, be pressed onto the 

 periphery of the blastoderm with a needle, 



