DEVELOPMENT AND REPRODUCTION 329 



§2. Influence of Internal Conditions on Development. — Tn the phys- 

 iological study of plant development and of the conditions con- 

 trolling this process, internal as well as external conditions must of course be 

 considered. The existence of a polarity in stems, for example, was demonstrated 

 by Vochting 1 in the following manner. Cut pieces of a willow shoot are sus- 

 pended vertically in a moist chamber, some of them being inverted, so that the 

 end that was originally toward the root (the root-pole) is now uppermost. The 

 upright pieces form roots at the lower end and leafy shoots at the upper, while 

 the inverted pieces develop only roots at the upper end — where these organs 

 seem to be teleogically useless— and only branches at the lower end (Fig. 170). 

 It thus appears that each piece of willow stem possesses two poles, a root-pole 

 and a shoot-pole, and the tissues near each pole always produce the kind of 

 organ characteristic of that particular pole, without reference to external con- 

 ditions, such as gravitation, light, etc. 



The mutual influence of various organs and their peculiar and seemingly 

 purposeful activities in the developmental process — in fact, all that is implied 

 in the term consensus partium — have long constituted an enigma in physiological 

 science. In animals, the regulating activities by which the correlation between 

 different organs and tissues are brought about have been, until recently, ascribed 

 to the nervous system, but it is now known that there are special substances 

 that control the activities of the different organs and even bring about the 

 development of new organs. Each of these substances is formed in some 

 special part of the organism and is then transferred to other parts, which may 

 be at a great distance, and it may there induce various kinds of chemical reactions. 

 Starling 2 has introduced the term hormone for this kind of substance, which 

 acts, as it were, like a chemical messenger. The effect of the development of one 

 organ upon that of another was emphasized, for the animal organism, by Brown- 

 Sequard, who showed that there is a chemical substance in the testes of the male 

 that affects the whole condition and even the mentality of the organism. His 

 conclusions concerning the influence of these substances are embodied in the 

 following quotations. "Je crois encore qu'il est parfaitement possible de reparer 

 des ans les outrages reparables." 3 "Les testicules donnent ä l'homme ses plus 

 nobles et ses plus utiles attributs." (Brown-Sequard, 1889, page 652.) 



Substances are produced in the testes by internal secretion, and these, 

 being distributed through the body, occasion many of the pronounced differ- 

 ences between male and female animals. There are many illustrations of 

 hormone action in animal physiology, of which a single instance may be referred 

 to in detail here, the relation between pregnancy and the development of the 

 lactiferous glands. Ribbert 4 grafted a mammary gland from one animal on to 



1 Vochting, Hermann, Ueber Organbildung im Pflanzenreich, i and 2 Th. Bonn. 1878 and 1884. 



'•' Starling, E. H., The Croonian Lectures on the chemical correlation of the functions of the body. 

 Lancet 169: 339—341. 423-425. 501-503, 570-583. 1905- Bayliss and Starling, 1906. [See note 2, p. 

 i-o.l 



3 Brown-Sequard, С E., Experience demonstrant la puissance dynamogenique chez l'homme d'un liquide 

 extrait de testicules d'animaux. Arch, physiol. 1 : 651-658. 1889. 



* Ribbert, Hugo, Ueber Transplantation von Ovarium, Hoden und Mamma. Arch. Entwickelungs- 

 mech. der Organismen 7: 688-708. 1898. 



