90 SPECIFIC EXAMPLES OF SLIDE-MAKING 



The next operation is to cut the embryo from the yolk by a series of cuts 

 made well outside the terminal blood vessel, which marks the limits of the 

 developing embryonic structures. To do this with success requires more cour- 

 age than experience. Just as soon as the vitelline membrane is punctured, the 

 yolk starts squirting out through the hole, rendering the fluid milky so that 

 one can no longer see the embryo. The smaller the hole which is cut, the 

 more violently does the yolk squirt out, so that the larger the scissors which 

 can be employed, the more easily will the embryo be removed. The easiest 

 method is to take blunt forceps in the left hand and with it grip the extra- 

 embryonic areas of the chick well outside the sinus terminalis. A certain 

 amount of drag is placed on it so that the vitelline membrane is wrinkled, and 

 then with a large pair of scissors a transverse cut is made directly away from 

 the operator about a third of an inch outside the sinus terminalis on the side 

 of the embryo opposite 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 be run on each side of the 

 embryo. The part gripped with the forceps should be released, and the free 

 edge, where the first cut was made, gripped so that the embryo can be folded 

 back away from the yolk. It is now relatively easy to sever all connection 

 between the embryo and the underlying materials by a fourth cut. The embryo, 

 held by the forceps in the left hand, will now be floating free in the saline 

 solution. The embryo is much stronger than it looks and will not be damaged 

 provided the tips of the forceps are kept under the saline solution. 



The embryo must be transferred to clean saline solution, preferably in 

 another finger bowl. This transfer may be made either with a very wide- 

 mouthed pipette of the eye-dropper type or by scoopmg it up in a smaller 

 watch glass with plenty of saline solution and transferring it to the fresh solu- 

 tion. Here it should be picked up again by one corner with the forceps and 

 waved gently backward and forward to remove the adherent vitelline mem- 

 brane (which may have fallen off already) as well as such yolk as remains. At 

 this stage the embryo should be examined to make sure that the heart is 

 beating and that it is in fit condition for fixation. 



The embryo is now scooped out on one of the Syracuse watch glasses with 

 as little water as possible. Next it is necessary to persuade it to flatten on the 

 bottom in an upside-down position; that is, so that the portion of the embryo 

 which was previously in contact with the yolk is now directed toward the 

 operator. To determine which side of the embryo is uppermost requires con- 

 siderable 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 



