620 THE PHYSIOLOGY OF REPRODUCTION 



by Leo Loeb.^ He states that the presence of well-preserved corpora 

 lutea does not lead to proliferation in the mammary tissue until a 

 comparatively late stage of pregnancy (namely, after the twenty- 

 fourth day). He remarks that the guinea-pig appears to differ from 

 the rabbit in this respect, in that in the forme? animal the stimulus 

 has to accumulate for a considerable period before reaction sets in. 

 But in guinea-pigs castrated during pregnancy, and in which the 

 pregnancy continued, proliferative changes were absent in the 

 mammary gland. Complete extirpation of the corpora lutea similarly 

 prevents the active secondary proliferation of the i glands which 

 occurs at a late stage.^ The rat differs from the guinea-pig in that 

 the corpus luteum persists for a considerable time during lactation, 

 and perhaps consequently ovulation and heat are inhibited during 

 the nursing period of the rat, while in the guinea-pig they continue 

 to take place. The effects of castration pn the lactating glands of 

 the rat and guinea-pig are not noticeable until suckling ceases, when 

 involution sets in. With regard to the nature of the stimulus calling 

 forth lactation Loeb says it is neither purely functional nor purely 

 formative but intermediate between the two.^ 



Steinach's experiments in which he transplanted ovaries into 

 previously castrated guinea-pigs or rats are referred to elsewhere 

 (see p. '346). He concludes that all the influence exercised by 

 the ovary on the mammary glands is due to the interstitial or 

 puberty gland, which promotes mammary growth and secretion. 

 Athias* has described similar experiments on guinea-pigs, and, as 

 a result of studying the histology of the transplanted ovaries, is 

 of opinion that the development of the mammary glands and the 

 initiation of milk secretion depend upon hypertrophied theca cells 

 or ripe follicles. Both these investigators, therefore, contrary to 

 the views of Aneel and Bouin and others, conclude that the presence 

 of corpora lutea is unnecessary for the development of the mammary 



' Loeb and Hesselberg, " The Cyclic Changes in the Mammary Gland, etc.," 

 Jmir. Exp. Med., vol. xxv., 1917; Loeb and Kuramitsu, "The Influence of 

 Lactation," "The Involution of the Uterus, etc.," and "The Effect of Suckling," 

 Atner. Jour. Physiol., vols. Iv. and Ivi., 1921 ; and Loeb, " The Relation of the 

 Ovary to the Uterus and Mammary Gland," Trans. Amer. Gyn. Soc, 1917. 



^ Extirpation of corpora lutea in the guinea-pig, according to this author, 

 accelerates ovulation and heat and the associated primary proliferation of the 

 mammary gland. 



3 A functional stimulus is one leading to activity unaccompanied by growth ; 

 a formative stimulus leads to a multiplication of cells and nuclei, i.e. to growth 

 generally and to differentiation (Virchow). In the mammary gland secretion 

 is accompanied by amitotic nuclear division. Mitotic proliferation precedes 

 and follows secretion, but is not associated with actual secretion. 



* Athias, "L' Activity S6cr6toire de la Glande Mammaire Hyperplasi^e," 

 C. R. de la Soc. de Biol., vol. Ixxviii., 1915; "Etude Histologique d'Ovaires 

 Greff6s," and " Sur le D6terminisme de I'Hyperplasie," C. R. de la Soc. de Biol., 

 vol. Ixxix., 1916. 



