16 BIOTIC STRUCTURE AND BIOTIC ENERGY 



as their outward histological expression increase in volume of the 

 cells, the obscuration of the cell outlines and nuclei, and the appear- 

 ance of discrete granules packing the cells ; when this has proceeded 

 to a certain extent the gland commences to discharge, either be- 

 cause of an adequate stimulus by nerve impulse renexly reaching 

 it, or by hormone in the circulation arising from some other physio- 

 logical chemical process preceding it in order of time, or in absence 

 of these usual causes by a slightly higher charge leading to spon- 

 taneous discharge, and the chemical processes are reversed in 

 the metabolism of the colloids. The same effects are seen 

 even more typically in the case of the mammary glands during 

 lactation. 



The whole of the physiological and chemical processes of re- 

 production in the mammalia, as well as in other lower animal classes, 

 illustrate the phasic activity of living matter in a most striking 

 fashion. In the lower mammalia, there are the seasonal periods of 

 sexual activity during which in ordered sequence the whole repro- 

 ductive system passes into excitement in its different parts, showing 

 the pre-oestral period, then the active mental and other excitement 

 of the osstrus, followed in turn by the post-osstral period. In the 

 human being, there is the monthly variation in the sexual activities 

 affecting ovaries and mammary glands, and then the uterus in 

 menstruation refreshing the uterine mucous membrane, followed 

 by the period of sexual desire, and this again by the period of sexual 

 quiescence completing the cycle. 



If fertilisation occurs, there is during gestation the modified 

 disturbance at each monthly interval during gestation, culminating 

 normally at the tenth monthly cycle with the expulsion of the 

 matured foetus. 



Similar phasic variations connected with the reproductive pro- 

 cesses are seen throughout the whole animal world, and even 

 become more striking the lower the organism is placed in the scale. 

 Witness the sexual metamorphoses in the insecta, and the develop- 

 mental history of echinodermata and mollusca. Also in protozoa 

 there are the alternations of sexual and asexual cycles determined 

 apparently by alterations in chemical constitution, and capable of 

 induction by variations in the environment. 



In the vegetable world there are the same phasic variations in 

 the activities of the cells accompanying the alternations of the 

 seasons, which have as their basis alterations in chemical equilibrium 



