An interesting observation on the rôle of internal secretion in growth was made by Leo Loeb.[149] When the fertilized ovum comes in contact with the wall of the uterus it calls forth a growth there, namely the formation of the maternal placenta (decidua). This author showed that the corpus luteum of the ovary gives off a substance to the blood which alters the tissues in the uterus in such a way that contact with any foreign body induces this deciduoma formation. The case is of interest since it indicates that the substance given off by the corpus luteum does not induce growth directly, but that it allows mechanical contact with a foreign body to do so while without the intervention of the corpus luteum substance no such effect of the mechanical stimulus would be observable. The action of the substance of the corpus luteum is independent of the nervous system, since in a uterus which has been cut out and retransplanted the same phenomenon can be observed. [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.”