276 



METHODS AND FORMULAS 



DS 11.10 



two to three inches depth, a number of 

 Syracuse watch glasses, a large quantity 

 of a 0.9% solution of sodium chloride, a 

 pair of large dissecting scissors, a pair of 

 fairly fine forceps, a pipet of the eye-drop- 

 per t.ype, some coarse filter paper, and a 

 pencil. No ver}' great accuracy is required 

 in making up the normal salt solution, 

 although it is customarily specified that 

 the temperature of the solution should be 

 102 to 103°F. Anywhere within 10 degrees 

 on either side of these figures, however, is 

 relatively safe. 



The egg is removed from the incubator 

 and placed in one of the fingerbowls which 

 is then filled with enough warm normal 

 saUne to immerse the egg completely. If 

 the operator is rather skilled, it is, of 

 course, possible to break the egg into the 

 warm saline as though one were breaking 

 it into a frying pan. But it is recom- 

 mended that the inexperienced worker 

 prepare several hundred embryos before 

 attempting to do this. The method by 

 which he can be assured of securing a per- 

 fect embryo on every occasion is first to 

 crack open the air space which lies at the 

 large end of the egg and then to let the air 

 which hes within it bubble out through the 

 warm saUne. This permits the yolk to fall 

 down out of contact with the upper sur- 

 face of the shell, which may now be re- 

 moved, as he works from the air space 

 toward the center with a pair of blunt-nose 

 forceps. Again a matter of practice is in- 

 volved, for a skilled operator can remove 

 this shell in large portions, while the inex- 

 perienced one should work very carefully 

 to avoid puncturing the yolk. If the yolk 

 is punctured it is simpler to throw the 

 egg away and start with another one. 

 After about half of the shell has been re- 

 moved, it will be quite easy to tip the yolk 

 with the embryo lying on top of it out into 

 the saline. 



The next operation is to cut the embryo 

 from the yolk by a series of cuts made well 

 outside the blood vessel sinus terminale 

 which marks the limits of the developing 

 embryonic structures. To do this with suc- 

 cess requires more courage than experi- 

 ence. Just as soon as the vitelline mem- 

 brane is punctured, the yolk starts 

 squirting out through the hole and render- 



ing the fluid milky so that the embryo is 

 obscured. The smaller the hole which is 

 cut, the more violently does the yolk 

 squirt out. Thus, the larger the scissors 

 which are employed, the more easily will 

 the embryo be removed. The easiest 

 method is to take a pair of blunt forceps 

 in the left hand and grip the extra-embry- 

 onic areas of the chick well outside the 

 sinus terminale. Use a certain amount of 

 drag, so that the vitelline membrane is 

 wrinkled, and then make a transverse cut 

 with a large pair of scissors directly away 

 from you, about a third of an inch outside 

 the sinus terminale on the side of the em- 

 bryo opposite to that which is held by the 

 forceps. This initial cut should be at least 

 an inch long and should be made firmly. 

 Two cuts at right angles to the first, each 

 an inch in length, should then be run on 

 each side of the embryo. The part gripped 

 with the forceps should then be released, 

 and the free edge where the first cut was 

 made should be gripped so that the em- 

 bryo can be folded back away from the 

 yolk. It is now relatively easy by a fourth 

 cut to sever all connections between the 

 embryo and the underlying materials. The 

 embryo, held by the forceps in the left 

 hand, will now be free in the saline solu- 

 tion. The embryo is much stronger than it 

 looks and will not be damaged provided 

 the tip of the forceps is kept under the 

 surface of the solution. 



The embryo must now be transferred 

 to clean saline, preferably in another fin- 

 ger-bowl. This transfer may be made 

 either with a very wide-mouthed pipet of 

 the eye-dropper type or by scooping it up 

 in a smaller watch glass with plenty of 

 saline and transferring it to the fresh 

 solution. Here it should be picked up 

 again by one corner with the forceps and 

 waved gently backward and forward. This 

 is to remove from it the adherent viteUine 

 membrane (which may, however, already 

 have fallen off) as well as to wash from it 

 such yolk as may remain. At this stage the 

 embryo should be examined to make sure 

 that the heart is beating and that it is in a 

 fit condition for fixation. 



The embryo is now scooped out on one 

 of the Syracuse watch glasses with as Uttle 

 water as possible. Next it is necessary to 



