CYCLES OF REPRODUCTION I5 



whether the breeding season is short or long it is possible 

 to conclude that a typical male is ready to breed only 

 once a year. 



In a non-pregnant female, however, cycles of lesser 

 duration are commonly found. These are most obvious 

 in the mammals where they are known as oestrous cycles, 

 and are seen as periods of heat, or oestrus, which recur 

 at intervals of days, weeks, or months according to the 

 species. Only during oestrus are the eggs shed from 

 the ovary, and only then does the female become 

 sexually receptive so that fertilization can occur. Thus 

 oestrus is characterized by both structural and behav- 

 ioural changes. The word itself refers to the behavioural 

 change and is derived from a Greek word meaning a 

 gadfly. By suggesting a resemblance to the agitation 

 caused by the bite of this fly, it indicates the excitement 

 that is often shown by an animal at this time. 



In the fishes and amphibians such lesser cycles appear 

 to be rare. Most females shed all their eggs quickly 

 during a period of days or weeks, and then spend the 

 rest of the year in building up a new stock. However, 

 as they come to be studied in sufficient detail some 

 species with prolonged breeding seasons may be found 

 to show lesser cycles. Bretschneider and de Wit*^ have 

 already described a kind of oestrous cycle in the female 

 bitterling, and Jaski^i^ has done the same in the little 

 viviparous millions fish. In the former species the cycle 

 is particularly obvious since it involves the periodic 

 growth of a prominent ovipositor. This takes place at 

 intervals of from about 7 to 14 days, and it is only at 

 these times that the eggs are laid.^^s 



Among birds it is a common habit for an egg to be 

 laid each day until the clutch is complete, and this may 

 be likened to a 24 hour oestrous cycle. Also during 

 spring and summer some species of birds lay two or 

 three clutches of eggs at intervals of perhaps 6 weeks. 

 However, this clutch cycle does not seem to be com- 

 parable to an oestrous cycle since it is now known that 



