the light from the microscope lamp behind it is 

 still in alignment when the bulb is taken away. 



If critical studies are to be made under low 

 magnification, or if photomicrographs are to be 

 taken, an ordinary light bulb is not adequate, and 

 one returns to the arc or ribbon filament lamp or 

 other light source which will provide critical or 

 Kohler illumination. The upper and lower 

 lenses of condensers on most research micro- 

 scopes can be separated — usually Ijy sliding or 

 screwing them apart. The upper element is 

 laid aside and the lower element is moved up or 

 down until the light source itself or the edge of 

 the diaphragm in front of it is sharply in focus 

 at the level of the objects on the slide. After the 

 condenser has been lowered as far as possible 

 (up to the point just short of touching the edge 

 of the mirror) it may still be necessary to elevate 

 the slide above the level of the stage by as much 

 as 1.5 cms., in order to bring the image of the 

 light source or its diaphragm into focus. With 

 another make, the condenser is moved down close 

 to the mirror but the slide does not have to be 

 elevated above the stage in order to bring the 

 light to a focus on the slide. 



Technicians accurately identified cell types 

 and their developmental stages when microscopes 

 provided with N. A. 1.3 apochromatic objectives 

 and N. A. 1.4 condensers were furnished and 

 when the liglit sources were carefully aligned 

 with the microscope. 



The illustrations shown in this Atlas were used 

 during the training of the technicians. 



PROCURING BLOOD 



Preparation of caunulas for taking Mood 

 from early embryos 



The problem of drawing out glass tubing to a 

 small diameter seemed on the face of it to be a 

 rather simple one, but it was soon discovered 

 that a broken, jagged tip would not smoothly en- 

 ter the dorsal aorta of embiyos, 2 to 3 days old, 

 or the tip of the heart of older embryos. The 

 task called for a delicate, almost microscopic tip 

 with a smooth-beveled point like that of a hypo- 

 dermic needle. 



Pyrex tubing, 2 mm. inside diameter and 

 4.5 mm. outside diameter, was used. An area 

 in the center was heated with an oxygen-gas 



flame. The ends were pulled apart quickly in 

 order to give a rapid taper and to keep tlie walls 

 thin. As the tubing was- pulled, a 60 angle was 

 made between the thin center portion and the un- 

 healed ends. After a dozen or so of these had 

 Ijeen made, the thin portion was broken off sev- 

 eral centimeters from the point where the taper 



began. 



The next step was the preparation of a micro- 

 burner. Any burner will do if the flame can be 

 reduced to a height of about 2 mm. A glass 

 sleeve, tapered to a small opening, was placed 

 over the metal tube of a microburner, and this 

 gave a small flame. Even though the flame was 

 small, it was approached cautiously. While the 

 thin part of the tulte was heating a slight tension 

 was exerted so that as soon as the thin glass had 

 softened it could be pulled quickly to a thin, 

 short tip. If pulled too quickly the tip was 

 long and thin and not stiff enough to push through 

 the tissues of the embryo and, if one hesitated 

 too long, the walls tliickened and the bore be- 

 came excessively small. The best tips were se- 

 lected from those made and the delicate, thin 

 tubing was broken off about a half centimeter 

 from the portion of intermediate thickness. 



The next step was to grind the end to a bevel 

 tip. The method followed was not the best — 

 there was a certain amount of breakage — but 

 after studying the principles set forth one can 

 add refinements that will make the process easier. 

 A small highspeed motor that carried a three- 

 jawed chuck over the end of the shaft was used. 

 It was mounted vertically in a bracket and two 

 rheostats were hooked in series to the current 

 line. One rheostat of the correct type would 

 be sufficient if it could be regulated delicately 

 enough; the only requirement is that the motor 

 turn at a slow, constant speed. The speed was 

 not measured l)ut it was estimated to be 1 to 4 

 revolutions per second. Into the chuck was in- 

 serted a small spindle that carried near the end 

 a flat emery disk about an inch in diameter. 

 When the emery disk was in place and rotating 

 slowly, a flat dish of water was lifted up beneath 

 it so that the disk turned just below the surface of 

 the water. With the set-up described, the in- 

 creased friction often stopped the motor; prob- 

 ably a wonn-gear reducing unit would have given 

 greater power and a nearer constant speed. 



The fine glass tip of the cannula was lightly 

 touched to the flat surface of the moving emery 



225 



