LENGTH OF INCUBATION PERIOD 5 



Relation of the Length of the Incubation Period to Sex of Chicks 



It is desirable to know whether the length of the incubation period is in any way 

 associated with the sex of chicks. For purposes of study, table 2 was constructed 

 to show the percentage of males obtained in the seven different emergent periods. 



There is considerable evidence to indicate that the sex ratios were lower in 

 the chicks hatched through the second third of the 21st day of incubation. Chicks 

 emerging on the 22d day — periods 5, 6, and 7 — showed a high sex ratio. The 

 mean emergent period for all males was found to be 4.24, and for all females 4.09. 

 The difference in these values is small and does not appear to be of any great 

 significance. 



The data tend, however, to substantiate the postulate that females predominate 

 in the chicks emerging early and that males predominate in the chicks emerging 

 later. 



Relation of Length of Incubation Period to Viability 



Mortality rates for the first six months of life may be taken as a measure of 

 viability. These rates have been calculated for the first week, for one to four 

 weeks, between four and eight weeks, from eight to twelve weeks, and from twelve 

 weeks to housing time at about six months of age. Some chicks died on an 

 unknown date during the first eight-week period without the sex being known, 

 and are included as a separate group in the table. Total mortality for six months 

 is also included. Table 3 gives the summarized results. 



Mortality for the first four weeks showed essentially no relationship to the 

 time of emergence from the shell. Between the ages of four and eight weeks, 

 however, there appeared to be an important relationship between the length 

 of the incubation period and mortality rate. During this period the chicks in 

 emergent groups 1 and 2 showed no mortality; the chicks that hatched during 

 the last two periods of the 21st day (groups 3 and 4) showed considerable mortality; 

 and the chicks emerging in the last three periods, that is on the 22d day, showed 

 excessive mortality. 



There were 68 chicks missing at the end of the first eight weeks so that the 

 age at death as well as the sex was unknown. These chicks were placed in their 

 respective emergent period and the mortality recorded in the fifth line of the 

 table. No relationship between emergent period and mortality rate was ob- 

 served in this small group, except that the very late emerging chicks had a mortali- 

 ty rate of about 4.5 percent compared with about 1.5 percent for the chicks 

 from the other emergent periods. 



Mortality rates are recorded separately for the sexes after eight weeks. No 

 consistent relationship between time of emergence and mortality rate appeared 

 during the age period from eight to twelve weeks in either sex. Between twelve 

 weeks and six months of age, males had a significantly higher mortality rate than 

 females, but the relationship between time of emergence and mortality rate is not 

 conspicuous if it exists at all. The data do seem to indicate that both males and 

 females from the last emergent period were low in viability. The data in table 3 

 appear to indicate that chicks emerging late are very likely to exhibit low viability 

 between four and eight weeks of age and not at other ages during the first six 

 months. 



The last line of the table records the total mortality to the age of six months, 

 obtained by adding together the preceding mortality rates. The data indicate a 

 consistent increase in mortality rate with each eight-hour increase in length of 



