l8 VERTEBRATE REPRODUCTIVE CYCLES 



of the act of copulation that causes the ovary to release 

 its eggs. I'' 1 Thus a non-pregnant rabbit can be regarded 

 as experiencing perpetual oestrus. This is made possible 

 by the development of overlapping crops of eggs, each 

 crop developing, remaining mature for some 7 to lo 

 days, and then degenerating. Copulation occurring at 

 any time causes the shedding and fertilization of those 

 eggs which are then mature so that pregnancy after 

 mating becomes a certainty. 



6. CONCLUSIONS 



It has been stressed that within the life span of an 

 individual animal there are usually greater and lesser 

 cycles of reproductive activity, although a few species 

 such as the eel, which breeds only once at the end of 

 its life, may be quoted as exceptions. 



In all parts of the world sexual activity is commonly a 

 seasonal phenomenon. The seasonal cycle is normally of 

 a year's duration, but the actual time of breeding varies 

 from species to species, and may be in any month. 

 Within a single species, or group of related species, it 

 also varies widely according to latitude. 



The duration of the breeding season also varies 

 widely; In some animals like the common frog all the 

 eggs and sperm are shed in the course of a few days or 

 even hours, while in others like the wild rabbit breeding 

 may be prolonged for six or more months. 



One of the two groups of animals in which the breed- 

 ing season is not clearly defined is that which lives in 

 houses. From man himself to the uninvited house mouse 

 the habit of living a protected indoor life seems to have 

 resulted in an extension of the sexual period until in 

 many cases breeding has become continuous throughout 

 the whole year. The other group which is characterized 

 by continuous breeding is that of the primates (monkeys, 

 apes and man). Man is the one species common to both 

 these groups. 



