THE REPRODUCTIVE SYSTEM 139 



(Squirrels and Dormice, etc.), Marsupials (Phascologale), 

 and in the nest the offspring are nursed during their most 

 helpless stage. But nest-building is only a temporary 

 expedient in mammalian evolution, and reduction of the 

 number of young produced at a birth is the ultimate 

 outcome in a truly arboreal life. This is the result that 

 has been arrived at in the Primate stock. The terrestrial 

 Insectivora produce large families, Centetes even having 

 a litter of twenty; Tupaia in its arboreal nest still begets 

 three or four offspring at a birth. The family of the 

 Marmosets very constantly numbers three. Among the 

 Lemurs two young may be born at a time, and among 

 all the Monkeys one offspring is the general rule, though 

 two are not infrequently born. 



Multiple pregnancies are, of course, primitive; and 

 reduction of the family is acquired under the conditions 

 of arboreal life. This reduction of the family produces 

 its changes in the reproductive system. In the first 

 place, the prenatal nidus designed to accommodate, say, 

 twenty embryos may well be expected to show a structure 

 anatomically different from one that is designed to accom- 

 modate a single embryo. In an animal in which the 

 pregnancy is habitually multiple, the genital tract is 

 divisible into three distinct parts (see Fig. 51). Leading 

 from each ovary, from which the egg cells are shed, there 

 are, on each side of the body, (1) thin tortuous tubes 

 (the Falloppian tubes), which are merely ducts down which 

 the egg cells pass into (2) the uterine cornua, which form 

 the bilateral nidus in which the fertilized egg cells develop 

 into the embryos; these two uterine cornua meet in 

 (3) a small common median chamber, the body of the 

 uterus, which opens into the vagina, and so to the ex- 

 terior. In animals which have multiple offspring the 

 embryos are developed in the uterine cornua. In Cen- 

 tetes, which we have already instanced, ten embryos 

 might be expected in one cornu, and ten in the other. 

 As the family becomes reduced so do the uterine cornua 



