^Qg PROCURING AND CARE OF LIVING MATERIAL 



incubator is not provided with forced draft, the temperature should be regulated at about 

 100° F. (37. 5°C. ). Since the heated air tends to rise, the hanging thermometer should 

 have its mercury bulb at the level of the eggs. Each inch above the eggs the temperature 

 may register at least a degree higher than at the egg level. Once controlled incubation 

 has begun it should not be interrupted if normal development is desired. 



Humidity control: A relative humidity of 60% is best, although higher humidity is not 

 deleterious. In the smaller incubators several finger bowls of water should be placed 

 among the eggs. In the larger incubators there are generally large pans beneath the eggs 

 which pans should be filled with sand and kept constantly moist. Dehydration is one of 

 the commonest contributors to lethality. 



Rotation of the eggs: The hen generally turns the eggs frequently. This is not necessary 

 for the first day or two but thereafter all eggs should be turned at least twice daily in 

 order to prevent adhesion of the membranes. Rotation of operated eggs will, of course, 

 be limited. 



The candling of eggs: Incubation time is not an accurate criterion of ontogenetic age so 

 that it is necessary to provide a device for sending light through the blastoderm so that 

 its age can be approximated by direct observation. The mail order houses offer inexpen- 

 sive candling equipment but it is very simple to make one. Fasten a light socket to a 

 board; place around the socket a large tin funnel cut to fit, which acts as a reflector; in- 

 vert over the 100 watt bulb a small waste basket through the bottom of which is cut a 

 circular hole; fasten beneath the hole a piece of coarse wire gauze, somewhat depressed, 

 to cup an egg; cover the hole (outside of basket) with a heavy felt cloth (to cut out extra- 

 neous light) and cut a slit in its center slightly shorter than the length of the average egg. 

 A switch may be provided for the light, fastened to the base board. The entire cost 

 should be about 50 cents. The egg is placed over the slit in the black cloth, and will be 

 caught by the wire screen, and a strong light will come up through the egg so that the 

 blastoderm can be seen directly. It must be remembered that the 100 watt bulb gives off 

 a great deal of heat so that the examination should not be extended for long. A dark room 

 for the candling is best. 



The blastoderm can be seen by 48 hours and the yolk-sac circulation by 60-72 hours. 

 Thereafter, movements of the embryo and the interlacing extra-embryonic circulation 

 become increasingly apparent. Embryos which die early generally show coagulation of 

 blood in the sinus terminalis (blood ring). From the seventh to the thirteenth days the 

 chorioallantoic circulation can be seen, but after the thirteenth day the embryos appear 

 more and more opaque, accentuating the air space at the blunt end. Embryos which die 

 during the latter half of incubation show an indistinct air space demarcation. 



There are two peaks in the mortality curve of incubated hen's eggs, one about the third 

 or fourth day and the other just before hatching, when the extra-embryonic membranes 

 are drying up. Under the most ideal conditions even the best eggs show 3% mortality on 

 the fourth day, 15% on the 19th day, and 7% mortality during the balance of the incubation 

 period of twenty-one days (Romanoff, 1931). 



Egg mortality : While eggs may be layed the year around, under controlled conditions, the 

 best season for high level of fertility and hatchability is between February and June. Dur- 

 ing the heat of the summer, and due to exposure to excessive cold in the winter, other 

 periods may not prove as successful. An .average hatching value of 80% should be con- 

 sidered as very good. 



Staging of the chick embryo : For uniformity it is well to accept the designated chick 

 stages in Hamilton's revision of Lillie's "Development of the Chick". A similar series is 

 now available for the duck embryos in Koeckes 19 58 paper. (See pages 410,411,413) 



