1148 



SUBMAMMALIAN VERTEBRATES 



but no evidence is available as to the nature 

 of this hormone. Fraps (1942) showed that 

 premature induction of ovulation with LH 

 also resulted in premature expulsion of the 

 oviducal egg. This means that either LH 

 per se caused the expulsion or that the ovula- 

 tion liberated a hormone which caused ex- 

 pulsion of the egg. "Spontaneous" ovulation 

 is preceded by increased activity of the fun- 

 nel of the oviduct, as was shown in a film 

 by Warren and Scott. It is possible that 

 such activity also causes contractions in 

 the shell gland that would result in ovi- 

 position. That this is not the only mecha- 

 nism involved in oviposition is evident 

 from the terminal oviposition in a clutch 

 which is not accompanied by ovulation. Be- 

 fore oviposition takes place, the egg is turned 

 180° on its short axis so that it is laid 

 blunt end first (Bradfield, 1951). The find- 

 ings by Olsen and Byerly (1932) that about 

 80 per cent of the eggs are laid small end 

 first, were criticized by Bradfield ( 1951 ) . 

 He states that normally the bird gets uji, 

 turns the egg in the shell gland, settles again, 

 and lays the egg about one hour later. Ac- 

 cording to Bradfield (1951), Olsen and By- 

 erly (1932) disturbed the hens when they 

 picked them up to determine the orientation 

 of the egg at oviposition and thus prevented 

 the egg from being turned. Bradfield used 

 radiographic examination which did not dis- 

 turb the hens. 



Fraps (1955b) has presented a detailed 

 study of the time relationships between ovu- 

 lations, between ovipositions, and between 

 ovulations and ovipositions. His termi- 

 nology will be used here. This discussion 

 will be largely devoted to the relationship 

 between events in the egg laying cycle of 

 chickens kept under 14 hours of light, from 

 6 a.m. to 8 p.m. The oviposition cycle is the 

 number of consecutive days on which ovi- 

 position occurs plus the number of days on 

 which oviposition fails to occur before its 

 resumption. 



In the following equation some of the re- 

 lationships are presented : 



/ = n/in + z) 3 



in which / = oviposition frequency within 

 a cycle, n = number of days on which ovi- 

 position occurs (singly or consecutively), 



z = number of days intervening before ovi- 

 position is resumed. The discussion will be 

 mainly concerned with those cycles in which 

 z = 1, the so-called closed cycles. Using 

 Fraps' terminology, the consecutive ovi- 

 positions of a cycle may be called 

 Ci , ... €„ . The difference in time of 

 day when successive ovipositions occur 

 has been called "lag"; total lag is the 

 difference in time of day between first 

 and last oviposition of a clutch ; mean lag is 

 total lag divided by {n — 1). Fraps calcu- 

 lated the mean lag from a large number of 

 observations in which birds were exposed to 

 artificial light from 6 a.m. to 8 p.m. From 

 these calculations the following became ap- 

 parent: (1) the lag of the terminal oviposi- 

 tion is greater than at preceding places in a 

 clutch; (2) this lag decreases as n increases; 

 (3) lag for positions between the initial and 

 terminal ovipositions decreases as n in- 

 creases and this lag may approach zero for 

 large n. 



These facts for the oviposition clutch have 

 a bearing on the ovulatory cycle. Warren 

 and Scott (1935) established that ovulation 

 occurs within 14 to 75 minutes after ovi- 

 position of an egg, except in case of the last 

 oviposition of a clutch. Ovulation, however, 

 is not caused by oviposition. When the egg 

 in the oviduct is broken so that it is ex- 

 pelled prematurely ovulation does not oc- 

 cur prematurely (Warren and Scott, 1935). 

 Premature expulsion of an egg caused by in- 

 jection of posterior pituitary extracts is not 

 followed by premature ovulation. On the 

 other hand, oviposition can be induced pre- 

 maturely by inducing premature ovulation 

 with LH (Fraps, 1942). 



Fraps (1955b) calculated the regression 

 of interval between oviposition and ovula- 

 tion on mean lag between successive eggs. 

 The regression showed that for each in- 

 crease in mean lag of 1 hour the mean inter- 

 val between oviposition and ovulation in- 

 creased about 10 minutes. By considering 

 these characteristics (Fraps, 1955b for de- 

 tails) for ovulation and oviposition clutches, 

 Fraps observed that the lag between the 

 first and second ovulations of a clutch was 

 of the same order of magnitude as the lag 

 between penultimate and ultimate oviposi- 

 tions in the clutch. A consideration of lags 



