MAMMARY GLAXD AND LACTATION 



591 



therefore be assumed and used as a point 

 of departure for the present account which 

 can most profitably be concerned mainly 

 with developments which have occurred 

 since the last edition was published. Refer- 

 ence will freciuently be made to these re- 

 views in which authority will be found 

 for the many ex cathedra statements that 

 will be made, but original sources will be 

 cited wherever appropriate.^ 



As an aid to logical treatment of the sub- 

 ject the scheme of classification proposed 

 by Cowie, Folley, Cross, Harris, Jacobsohn 

 and Richardson (1951) will be followed in 

 this chapter. Besides introducing a system of 

 terminology in respect of the physiology 

 of suckling or milking, these writers have 

 put forward a classification scheme which 

 is an extension of one previously proposed 

 by one of the present authors (Folley, 

 1947). This scheme considers the phenom- 

 enon of lactation as divisible into a number 

 of phases as follows: 



[ [Milk synthesis 



I Milk secretion ■! Passage of milk from 

 I I the alveolar cells 



Lactation<J [Passive withdrawal of 



ij milk 



JThe milk-ejection re- 

 [ Hex 



Milk removal 



I 



As is logical and customary, discussion of 

 lactation itself will be preceded by consider- 

 ation of mammary development. 



II. Development of the Mammary 

 Gland 



A. HISTOGENESIS 



References to the earlier work on the 

 histogenesis of the mammary gland in vari- 

 ous species will be found in Turner ( 1939, 



^ Within the last 10 years there have been 

 several symposia devoted to the problems of the 

 physiology of lactation. The proceedings of these 

 symposia have been published: Mecanisme physi- 

 ologie de la secretion lactee. Strasbourg, 1950, 

 Colloqvies Internationaux du Centre National de 

 la Recherche Scientificiue. XXXII, 1951, Paris; 

 Svmposium sur la physiologie de la lactation, 

 Montreal, 1953, Rev. Canad. Biol., 13, No. 4. 1954; 

 .Symposium sur la physiologie de la lactation, 

 Brussels, 1956, Ann. endocrinol. 17, 519; A Discus- 

 sion on the Physiology and Biochemistry of Lacta- 

 tion. London. 1958, Proc. Roy. Soc, .ser. B, 149, 

 301. 



1952,) and Folley (1952a). There have also 

 been studies on the opossum (Plagge, 1942) , 

 the mouse and certain wild rodents (Ray- 

 naud, 1949b), the rhesus monkey (Speert, 

 1948), and man (Williams and Stewart, 

 1945; Tholen, 1949; Hughes, 1950). 



A question which in the last decade has 

 been receiving attention is whether the pre- 

 natal differentiation and development of the 

 mammary primordium is hormonally con- 

 trolled. According to Balinsky (1950a, b), 

 the mitotic index of the mammary bud in 

 the embryo of the mouse and rabbit is lower 

 than that of the surrounding epidermis and 

 he concludes that differentiation of the bud 

 is due not to cellular proliferation (growth) 

 but to a process of aggregation ("morpho- 

 genetic movement") of epidermal cells. This 

 author also reports that for some time after 

 its formation, the mammary bud is cjuies- 

 cent as regards growth, thus exhibiting 

 negative allometry compared with the whole 

 embryo, until the sprouting of the primary 

 duct initiates a phase of positive allometry. 

 The cjuestion is, what is the stimulus re- 

 sponsible for the onset of this allometric 

 phase? Is the growth and ramification of the 

 duct primordium, like that of the adult duct 

 system, due to the action of estrogen ema- 

 nating from the fetal gonad or from the 

 mother? 



Hardy (1950) has shown that dift'erenti- 

 ation and growth of the mammary bud of 

 the mouse could proceed in explants from 

 the ventral body wall of the embryo, cul- 

 tured in vitro, even when no primordia 

 were present at the time of explantation 

 (10-day embryo). Primary and then sec- 

 ondary mammary ducts and a streak canal 

 differentiated and a developmental stage 

 similar to that in the 7-day-old mouse could 

 be reached. Balinsky (1950b) was also able 

 to observe the formation and growth of 

 mammary buds in approximately their nor- 

 mal locations in a minority of cases in which 

 body-wall explants of 10-day mouse em- 

 bryos were cultivated in vitro. Discounting 

 the rather remote possibility that the effects 

 were due to minute amounts of sex hormones 

 present in the culture media, these observa- 

 tions indicate that hormonal influences are 

 not necessary for the prenatal stages of 

 mammary develo]iment, and in accord with 



