[149] Loeb, Leo, _Zentralbl. f. allg. Path. u. path. Anat._, 1907, xviii., 563; _Zentralbl. f. Physiol._, 1908, xxii., 498; 1909, xxiii., 73; 1910, xxiv., 203; _Arch. f. Entwcklngsmech._, 1909, xxvii., 89, 463; _Jour. Am. Med. Assoc._, 1908, l., 1897; 1909, liii., 1471.
Bouin and Ancel[150] have shown that the corpus luteum, which in the case of pregnancy continues to exist for a long time, is responsible for the changes in the mammary gland in the first half of pregnancy, when an active cell proliferation takes place in the gland. This process can be interrupted by destroying the corpus luteum artificially. During the second half of gravidity no further cell proliferation takes place, but the cells begin to secrete milk while during the period of cell proliferation such secretions do not occur.
[150] Quoted from M. Caullery, _Les Problèmes de la Sexualité_, Paris, 1913, p. 126.
Claude Bernard and Vitzou had shown that the period of growth and moulting of the higher crustacea is accompanied by a heaping up of glycogen in the liver and subdermal connective tissue. Smith[151] found that during the period between two moultings, when there is no growth, the storage cells are seen to be filled with large and numerous fat globules instead of with glycogen. He also found that in the _Cladocera_ "the period of active growth is accompanied by glycogen--as opposed to fat--metabolism." He observed, moreover, that if _Cladocera_ are crowded at a low temperature the fat metabolism (with inhibition to growth) is favoured, while at high temperatures and with no crowding of individuals the glycogen metabolism is favoured. In the latter case a purely parthenogenetic mode of propagation is observed, while in the former sexual reproduction takes place. The effect of crowding of individuals is possibly due to products of excretion, which then act on growth and reproduction indirectly by changing the "glycogen metabolism" to "fat metabolism."
[151] Smith, Geoffrey, _Proc. Roy. Soc._, B. 1915, lxxxviii., 418.
All these cases agree in this, that apparently specific substances induce or favour growth, not in the whole body, but in special parts of the body. Sachs suggested that there must be in each organism as many specific organ-forming substances as there are organs in the body.