48 MASS. EXPERIMENT STATION BULLETIN 391 



the sex ratio in lymphocytoma and led to an entireh' different result. Collection 

 C could not logicalK' be included, however, for the group was drawn from sources 

 entirely dissimilar from collections A and B. The literature contains no data 

 with which to compare these findings save the general comment previously 

 mentioned which has often been made, to the effect that no evidence suggests 

 a difference between male and female in incidence of lymphocytoma. These 

 statements are not accompanied by figures and hence provide no basis for com- 

 parison. The data from this survey suggest that the question should be ex- 

 amined and satisfactory information accumulated to provide a definite answer. 



Seasonal Occurrence 



Schneider (29) observed an increase in frequenc}' of tumor between Ma>' and 

 October and a decrease between November and Apri', which she associated with 

 the increased ovarian function of egg laying during the spring and summer. Dif- 

 ferent authors have discussed an apparent relation between season of the year 

 and the occurrence of leukosis in chickens (21). Other than these comments, 

 there is little information in the literature on seasonal occurrence of spontaneous 

 neoplasia in chickens. 



The distribution according to age and quarter of year at necropsy of 2304 birds 

 examined in the laboratory is given in Table 21. Comparable data on birds 

 affected with tumors are given in Table 22. The year was divided into four 

 quarters in which the first quarter included January, February, and March, 

 the second quarter the following three months and so on to the fourth quarter 

 which included October, November and December. Computations to determine 

 the significance of the findings become involved with many factors. 



Since the factor of age is of importance with respect to the incidence of the 

 various types of tumors, it must be considered in attempting to determine the 

 effect of season upon rate of incidence of tumors. The data on quarterly inci- 

 dence of seven types of neoplasia together with the total number of neoplasms 

 are given in Table 24. An attempt is made to compensate for the factor of age, 

 bv estimating the incidence of the respective types of neoplasia in only the age 

 group in which each type of tumor was actually encountered. Since the borders 

 of the age groups of birds examined do not exactly coincide in all instances with 

 the oldest and youngest ch'cken affected with tumor, the incidence rate is not pre- 

 ciselv accurate. The number of cases of neoplasia and number of birds examined 



Table 24. — Correlation Between Quarter of Year and Incidence of 

 Neoplasms Among the Chickens Examined. 



Incidence Expressed in Percent of 

 Age Group Group Examined and Found Affected 



Neoplasm of Birds 



Examined First Second Third Fourth 



(Months) Quarter Quarter Quarter Quarter 



Lvmphocvtoma 2-25+ 8.4 3.7 5.9 7.6 



Leiomyoma 7-25+ 1.28 3.9 5 1 1.08 



Embryonal nephroma . . 2-12 .22 .27 .92 1.23 



Myelocytoma 2-15 .87 .62 .85 .88 



Leukosis 4-25-f .49 2.10 .60 1.08 



Epithelioblastoma 2-25 -f .80 .59 45 1.11 



Fibrosarcoma 2-21 .21 1.03 .46 .81 



All neoplasms* 2-25+ 13.1 10.0 12.1 15.9 



♦Includes all those listed in Table 22. 



