134 Richard M. Fraps 



mechanisms embodied, for example, in nervous, neuroendocrine and endo- 

 crine controls over ovarian function. These mechanisms may indeed be 

 basically much alike in all avian (and mammalian) species, despite the great 

 and obvious diversity of reproductive patterns exhibited by dilTering species. 

 For the time being, therefore, it seems reasonable to suppose that ovulation 

 and related processes in the domestic fowl proceed from and are mediated 

 through mechanisms fundamentally similar in all avian species, however 

 peculiar or unique or even artificial some final expressions of reproductive 

 processes may appear to be. 



FOLLICULAR-PITUITARY RELATIONSHIPS 



Ovarian function in the fowl and in most birds differs in at least two 

 obvious respects from that of mammals, as Nalbandov (44) has recently 

 emphasized. The ovarian complement of developing follicles consists typically 

 of an array which exhibits well-defined differences in size, not of a group 

 comparable in size through successive developmental stages as is commonly 

 seen in mammals. Of the bird's complement, only a single follicle, the largest 

 of the series, matures and is ovulated at a time. In contrast, the simultaneous 

 maturation and ovulation of a number of follicles is typical of mammals. 

 Nalbandov has called attention particularly to the point that birds must be 

 assured of "an endocrine mechanism permissive of the existence of a hierarchy 

 of follicles of graded sizes, only one of which is capable of ovulating at any 

 one time". Experiments directed toward the imposition and maintenance of 

 the typical follicular hierarchy in the hypophysectomized hen by simulation 

 of the naturally occurring "endocrine mechanism" are discussed by 

 Nalbanov elsewhere in this volume. Of more immediate concern here are 

 relationships between follicular development, maturation and ovulation in 

 the intact hen, relationships which require, to begin with, some understanding 

 of timing of these processes. Various aspects of the subject have been dis- 

 cussed earlier (15-17, 37). 



The Ovulation Cycle 



Under optimal (12-14 hr) photoperiods, the hen typically lays an egg on 

 each of two or more consecutive days, does not lay on one day and then lays 

 again on two or more consecutive days. The eggs thus laid on consecutive 

 days constitute a sequence (often but wrongly called a clutch). In sequences 

 of low to moderate length (2 to about 8 eggs), the first egg is laid during early 

 morning (or lighted) hours, subsequent eggs at later hours on successive 

 days until the sequence is completed with lay of a terminal egg during 

 afternoon hours. The interval between successive eggs of a sequence is thus 

 somewhat greater than 24 hr; the term lag has been proposed (15) to describe 

 the difference between the interval separating lay of consecutive eggs and 



