116 MASS. EXPERIMENT STATION BULLETIN 211. 



hibit the pause, in contrast to only 30 per cent of those beginning late in 

 the season (December). It is possible that the appearance of the pause 

 is due to some direct eflfect of the season (length of days, for instance), 

 but since there is no uniformity in the length of the egg-la3'ing period, 

 and since one member of a flock may begin the second laying period at 

 the same moment another is finishing the first, it is clear that whatever 

 influence the environment may exert is secondary, the primary cause being 

 a change in the physiological condition of the layers, expressed in some 

 individuals by an actual pause, in others by a slowing down in production, 

 while in a few individuals no external effect becomes apparent. Note, as 

 shown in Fig. 13, that a flock of late-hatched pullets were laying at a high 

 level at the same time that their early-hatched sisters were in a slump. 

 Clearly it is not the environment alone that is responsible for the pause. 

 Some observations lead to the belief that environmental conditions which 



80 



o 60 



H 40 



-z. 



o 10 



SER 



OCT. 



NOV. 



DEC. 



JAN. 



FEB. 



Fig. 12. — Daily Production. 

 Heavy line 1917-18, Pen III; light line 1913-14. (See text for details.) 



at other times would not stop production maj^ do so in this sensitive 

 physiological state. Given an initial (winter) cycle of variable length, it 

 is apparent that in some individuals it may extend into early spring and 

 either overlap the spring cycle and thus fail to become apparent, or per- 

 haps, because of a direct stimulus due to longer daj'^s, production may be 

 kept up, and thus the winter pause is suppressed in pullets beginning to 

 laj'^ late in the season. 



As far as possible, selection has been directed against the winter pause, 

 and while not eliminated, there is evidence that its length has decreased, 

 and, correspondingly, the length of the initial period increased. This is 

 shown in Fig. 13, where a high production over a period of six weeks 

 was maintained, which is much longer than three years previously, as 

 seen in Fig. 12. The average number of eggs laid, prior to the pause, 

 was 12 more in 1920 than in 1917. 



