PHYSIOLOGY OF SEX AND REPRODUCTION. 



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stimulus of a new host admits of the development of the sexual tape- 

 worm. 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 amcricand), 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 occa- 

 sionally occurs. We have already referred to those dipterous midges 

 (Cecidomyice) , in which the larvae for successive generations become 

 reproductive, though only parthenogenetically. Very striking, too, 

 is the trematode worm Gyrodactylus, which recalls the mystical views 

 of the preformationists, 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 to sexual maturity. A 

 more marked precocity has been observed in the Alpine salamander 

 (Triton alpestris). In higher organisms, it occasionally happens that 

 long before growth has ceased or adolescence been reached sexuality 

 sets in, especially in the male sex, but this is fortunately a compara- 

 tively rare pathological occurrence. In one set of organisms pre- 

 cocious reproductive maturity has been of paramount importance, 

 namely, in the flowering plants. Here the prothallium stage, as 

 contrasted with the vegetative, has been much reduced, and has 

 remained associated with or been absorbed by the asexual genera- 

 tion. This is to be in part explained by the accelerated reproduction 

 of the prothallus, comparable to a similar process which has reduced 

 the separate medusoid sexual persons of a hydroid colony to mere 

 buds. 



III. Menstruation.-- The process of menstruation {menses, catamenia), 

 although from the earliest times the subject of medical inquiry, is by no means 

 yet clearly understood. It occurs usually at intervals of a lunar month in all 

 females during their period of potential fertility (fecundity), and so far from 

 being confined to the human species, has been observed at the period of 

 "heat" in a large number of mammals. Though thus clearly a normal 

 physiological process, it yet evidently lies on the borders of pathological change, 

 as is evidenced not only by the pain which so frequently accompanies it, and 

 the local and constitutional disorders which so frequently arise in this connection, 

 but by the general systemic disturbance and local histological changes cf 

 which the discharge is merely the outward expression and result. In general 

 terms, and apart from ovulation, menstruation may be described as a periodic 

 discharge of blood, glandular secretion, and cellular detritus from the lining 

 of the uterus. After from three to six days the blood ceases to appear, and 



