CONTROL OF ANNUAL GONADAL CYCLES 735 



also important to note that Lofts and Marshall (1956) have shown 

 that prolactin causes a testicular involution similar to postnuptial 

 metamorphosis in Passer domesticiis, Chloris cliloris, and Fr'mgilla 

 coelebs. Hypothetically then, it is possible to see how the sequence in 

 the production of pituitary gonadotropins might cause gonadal de- 

 velopment (and thence the reproductive behavior associated with 

 sex hormones), incubating and brooding behavior, development of 

 refractoriness, and regression of the gonads. 



In closing the discussion of the refractory period it is necessary to 

 direct attention briefly to the behavior of the testes during the refrac- 

 tory period. During this period (Marshall, 1949a, 1950, 1951, 1952, 

 1955) there is a fatty degeneration of the tubules and a loss of lipoid 

 materials from the cells of Leydig. The collapsed metamorphosed 

 testis soon forms a new interstitium and tunic and thereafter slowly 

 clears the tubules of debris from necrotic spermatozoa. Marshall has 

 referred to this period as a refractory period on the basis of his con- 

 clusion, from histologic studies, that at this time the testes cannot be 

 caused to develop. Obviously this testicular refractory period of 

 Marshall, although it may occur at the same time as the refractoriness 

 of the photoperiodic response mechanism, must be clearly distin- 

 guished from the period of refractoriness of the response mechanism 

 as discussed above. There is a real need for a careful investigation to 

 ascertain experimentally if there is a time in the period of testicular 

 metamorphosis when the testis cannot respond to the gonadotropins 

 from the anterior pituitary. 



PHOTOPERIODIC CONTROL OF OVARIAN CYCLES 



In general it appears that in those species in which photoperiodic 

 control can be demonstrated for the testicular cycle there is also a 

 photoperiodic control of the ovarian cycle. However, it is certainly 

 the experience of all investigators of passerine species that, although 

 males may be brought into full reproductive state (production of 

 spermatozoa) by simply increasing the daily photoperiod adequately, 

 such treatment alone causes only partial ovarian development. Thus 

 it appears that whereas the daily photoperiod may be the primary 

 timer, other factors are relatively more important and essential than 



