SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 243 



difficult to see how the ova once liberated into the body-cavity find their 

 way safely into the small opening of the duct. In the frog, however, 

 tracts of the peritoneal epithelium become ciliated, so propelling the ova 

 in the right direction. In reptiles, birds, and mammals the open end of 

 the oviduct is widened, fringed, and ciliated, and lies close to or even 

 touching the ovary ; muscular fibres too are present, and more or less 

 active movements of this cilated end over the ovarian surface have been 

 alleged to occur. The oviduct once reached, the downward progress of 

 the ovum is ensured by the ciHa of the epithelial lining, and probably also 

 by peristaltic movements of its muscular coat. 



There is no doubt that the advent of sexual maturity varies 

 with environmental conditions of climate, food, and the like. 

 Broadly speaking, sexuality becomes pronounced as growth 

 ceases. Especially in higher organisms, a distinction must 

 obviously be drawn between the period at which it is possible 

 for males and females to unite in fertile sexual union, and the 

 period at which such union will naturally occur or will result in 

 the fittest offspring. In the lower anijiials, where the individual 

 life is usually shorter, sexual maturity is more rapidly attained, 

 though we find cases such as that of the fluke {Polysfomiwi) so 

 commonly present in the bladder of the frog, where maturity of 

 the reproductive organs does not occur for several (three) years, 

 and maturity of growth for some years afterwards. In cestode 

 parasites, the bladder-worm stage remains indefinitely asexual, 

 until in fact the stimulus of a new host admits of the develop- 

 ment of the sexual tapeworm. In plants, reproductive maturity 

 sets in at various ages; thus we have all gradations, at the 

 one extreme our characteristically short-lived but magnificent 

 annuals, then the biennials, and from these to a maturation at 

 still longer date, as in the well-known case of the American 

 aloe {Aloe americana)^ which even in Mexico takes from seven 

 to twelve years to reach the floral climax in which it expires, 

 and in our greenhouses as much as a generation or two, whence 

 its name of "century plant." 



In contrast to such cases, precocious reproductive maturity 

 occasionally occurs-. We have already referred to those 

 dipterous midges {Cecidomyice)^ in which the larvae for succes- 

 sive generations become reproductive, though only partheno- 

 genetically. Very striking too is the trematode worm 

 Gyrodactyhis, w^hich recalls the mystical views of the prefor- 

 mationists, in exhibiting three generations of embryos, one 

 within the other, while the oldest is yet unborn. The well- 

 known axolotl of Mexican lakes, though with its persistent 

 gills in a sense the larval form of Amblystoma, attains of course 



