12 MASS. EXPERIMENT STATION BULLETIN 307 



Age of birds used does not consistenth' affect hatchability. Hays (1928) 

 presents some evidence that for identical matings there is a slight increase in 

 hatchability of \earlings compared with pullets. When different groups of 

 pullets, yearlings, and old hens are compared for mean hatchability, the station 

 records from 1924 to 1931 show the following means: 65 per cent for pullets, 75 

 per cent for yearlings and 65 per cent for old hens. In the experiment station 

 flock there is, therefore, some evidence indicating that females are likeh to hatch 

 the highest percentage of fertile eggs during their second la\ing >ear. 



The most potent factors affecting hatchabilit}' are inherited factors, according 

 to the work of Hays and Sanborn (1924). They have clearly demonstrated that 

 the hatching record of a hen depends largely upon her inheritance and upon the 

 character of the male to which she is mated. In other words, the embryo's 

 inheritance from its sire and dam either equips or fails to equip it to emerge 

 normally from the shell. 



Studies at the Massachusetts Station indicate that one major gene, H, is 

 concerned in high hatchability. This gene is transmitted equally b\- males and 

 females and has a cumulative effect. The three general classes of birds with 

 respect to hatchability are: (1) HH birds which, if mated to either HH or Hh 

 males, give hatching records of 85 per cent or more; (2) Hh birds which usually 

 have a hatchability range from 55 to 84 per cent and may produce no hatchable 

 eggs; and (3) hh birds which generalh- range in hatchability from to 54 per cent. 

 Females that have shown a hatchability of 85 per cent or more are HH in genetic 

 make-up, and breeding birds should always be selected on this basis. Males 

 should be chosen on the hatching record of their female ancestors and insofar as 

 possible upon the actual hatching records of their daughters. 



By giving some attention to hatchability in the station flock, the mean has 

 been raised from about 58 per cent in 1914 to a maximum of about 82 per cent 

 in 1933. A portion of this increase may be attributed to improved incubation 

 facilities, but in general there has been an upward trend under the same s>stem 

 of incubation. Attention should be called to the fact that all eggs have been 

 pedigree hatched which necessitates considerable extra handling and exposure 

 to temperature changes and has been shown to lower the hatching records from 

 10 to 15 per cent over ordinary incubation in this flock. 



The Vigor Problem 



High vigor in a production-bred flock may be defined as the ability to live and 

 produce heavily for a long period of time under intensive conditions. Records 

 of the mortality rate in families on the growing range and in pullets in the laying 

 houses show striking differences between families. It is a reasonable assumption 

 that the great proportion of these losses come from lack of vigor, because diagnos- 

 tic records show the flock to be relatively free from specific disease outbreaks. 

 The flock has been free from pullorum disease for man\- years, and pox and 

 roup have been controlled by vaccination. Coccidiosis has been held in check 

 by a four-year range rotation and by quarantining against visitors or caretakers 

 of adult fowls. 



From October 1, 1932 to September 30, 1933, complete post-mortem records 

 were made by the Department of Veterinary' Science on all pullets of the experi- 

 mental flock hatched in 1932. The records begin at the time the birds were 



