1134 



SUBMAMMALIAN VERTEBRATES 



other hand, are the result of the difference in 

 age at which the thyroid was removed or 

 the result of incomplete removal of all 

 thyroid tissue (especially because ectopic 

 thyroid tissue seems to occur sometimes) 

 cannot be judged from the few data now 

 available. It is interesting to note that, in 

 certain families of White Leghorn chickens, 

 birds without any apparent thyroid tissue 

 or with thyroids consisting of one abnormal 

 follicle occur. In the most severe cases of 

 hypothyroidism ovaries are immature at an 

 age when normal hens of the same strain are 

 in full production. Thyroxine injections will 

 bring such hypothyroid birds into produc- 

 tion in 7 to 21 days (the information on 

 these hypothyroid hens was communicated 

 to the author by Dr. R. K. Cole) . The bal- 

 ance between gonadotrophin and thyroid 

 hormone is apparently rather important. 

 Clavert (1958) noted, for instance, that the 

 ovarian response to P]MS was reduced by 

 simultaneous thyroxine injections of pi- 

 geons. Such an inhibition seems understand- 

 able in view of the inhibition of estrogen- 

 induced lipemia and proteinemia. In effect, 

 thyroxine decreased the concentration of 

 yolk precursors and thus lowered the re- 

 sponse to PMS. In view of this effect of 

 thyroid hormone, it is not surprising that 

 thyroid hormone feeding (mainly as iodi- 

 nated casein) has given opposite results in 

 different experiments. These results have 

 been reviewed recently (Turner, 1959; van 

 Tienhoven, 1959) and do not need to be 

 discussed here. 



8. XutritioJi 



The specific effect of nutrition on ovarian 

 activity has not been studied in great de- 

 tail; this is in contrast to the many studies 

 on the effect of nutrition of the hen on the 

 hatchability and embryonic development 

 (Cravens, 1949; Landauer, 1951). Restric- 

 tion of energy intake delays sexual matu- 

 rity (Bruckner and Hill, 1959). Pullets, 

 after being reared on a restricted diet, when 

 fed ad libitum at the approach of sexual 

 maturity, produced more eggs during the 

 rest of the year than pullets reared and 

 maintained on an unrestricted diet. The 

 mechanisms involved in these relationships 

 have not been studied. It seems probable 

 tliat restricted energy intake results in later 



gonadotroi^liin secretion, thus delaying sex- 

 ual maturity. 



^Yithdrawal of feed from laying hens re- 

 sults in atresia of the follicles and a simul- 

 taneous decrease in serum vitellin (Hosoda, 

 Kaneko, Mogi and Abe, 1955b). These ef- 

 fects can be prevented by injections of FSH 

 or PAIS (Hosoda, Kaneko, Mogi and Abe, 

 1955a), and follicles so maintained can be 

 ovulated (Hosoda, Kaneko, JMogi and Abe, 

 1956). The results suggest that starvation 

 prevents production of gonadotrophin, a 

 suggestion supported by bioassays of the 

 pituitaries. Phillips (1959) found that the 

 testes of chicks injected with pituitaries 

 from starved hens with atretic follicles 

 weighed 7.6 mg. compared with 10.11 mg. 

 for the testes from chicks injected with pi- 

 tuitaries from well fed, laying hens. The 

 difference was not statistically significant, 

 but shows a trend in the expected direction. 

 No data seem to have been published on the 

 specific effect of the separate nutrients on 

 ovarian activity. In most studies egg pro- 

 duction was measured and no efforts seem to 

 have been made to separate the effects of 

 inanition from the specific nutrient effect. 



C. REGULATION OF BREEDING CYCLES OF 

 SEASONALLY REPRODUCING BIRDS 



Aristotle's observation that the testes are 

 small during the nonbreeding season and 

 large during the breeding season bears wit- 

 ness to the accurate observations that have 

 been made throughout history. It does not 

 require much imagination to visualize that 

 man must have observed the effect of en- 

 vironment on reproduction. The regular 

 flight north of flocks of geese every year 

 must have impressed the hunting tribes. But 

 only recently some understanding has been 

 obtained of the pathways by which the en- 

 vironment can affect reproduction. 



1. Hypothalamic-Pituitary System 



In order to understand the explanations 

 proposed here, a review of the control of 

 the anterior pituitary is required. The dis- 

 cussion is i^urposely limited to birds, because 

 the relationships for other vertebrate classes 

 are described in the chapters by Greep and 

 Purves. 



It seems that the only manner in which 

 environmental stimuli, especially those such 



