INTERNAL SECRETION OF CORPUS LUTEUM 189 



still leaves the gland in a rudimentary condition and only during 

 pregnancy (and to a lesser degree during pseudo-pregnancy) 

 does the real mammary development occur. 



The initial development of the female mammary glands is 

 clearly endocrine in nature (see p. yS), and the control during 

 pregnancy must also be of the same nature since transplantation 

 of mammary tissue to abnormal sites does not affect its normal 

 development (534). The foetus and placenta, as well as the 

 ovary, have been suggested as possible sources of the stimulus 

 required during pregnancy, but a wealth of evidence has now 

 accumulated to show that the corpus luteum is the responsible 

 factor. 



The mammary gland in pseudo-pregnancy. After ovulation, 

 correlated with the development of the corpus luteum, an 

 entirely new phase of mammary growth sets in. The extent to 

 which this growth takes place in the non-pregnant animal varies 

 with the species, and with the intensity of the luteal phase in the 

 non-pregnant animal. In the unmated rat and mouse, where the 

 luteal phase of the cycle is missing, growth of the mammary 

 gland appears to take place only at oestrus (see p. 53). 

 Where the luteal phase becomes pronounced, as during 

 pseudo-pregnancy in Dasyurus, and in the rabbit and the ferret, 

 very considerable development of the mammary gl-and takes place 

 even in the absence of foetuses. The withdrawal of the stimulus 

 at the end of pseudo-pregnancy in these animals results in at 

 least a temporary secretion of milk. In the rabbit the develop- 

 ment of the mammary glands during pseudo-pregnancy and 

 during pregnancy has been studied in very considerable detail 

 by Anceland Bouin (30, 32) and Hammond (264). Even when 

 oestrus has lasted for some months the continued activity of 

 oestrus-producing hormone causes no development of the 

 mammary glands other than the slight growth normally 

 associated with oestrus. Immediately ovulation takes place, 

 however, and the corpora lutea are formed, development of the 

 mammary gland begins and even in the absence of pregnancy 

 continues for some fourteen days, that is, as long as the corpus 

 luteum of pseudo-pregnancy is functional. This proliferation 

 consists in the lateral extension and swelling of the ducts. 

 Clumps^ of alveoli also develop at the ends of the milk ducts. 



