STUDIES ON THE MAMMARY GLAND .S73 



about the fourth and fifth weeks. Whether there are definite 

 factors causing this first increase has not been determined. It 

 is possible that it is due to individual variation. It does not 

 appear to be due to any greater relative increase in the body 

 weight at this period, and is of doubtful significance. 



The second and more important period of increase in the mam- 

 mary gland occurs about the ninth week. Donaldson ('15) states 

 that the female rat arrives at the age of puberty about GO to 70 

 days after birth, the gonads indicating sexual maturity at the 

 age of two months or less. Jackson (12) however, found preg- 

 nancy to occur in one case at the age of seven weeks. Lantz 

 ('10) cites from Buckland a case where a white rat is said to 

 have given birth to 1 1 young at the age of eight weeks (and which 

 accordingh" must have become pregnant at the age of five weeks). 

 These are very exceptional cases, however. 



Jackson ('12) states that the vaginal aperture does not appear 

 until the middle or end of the second month. There is consid- 

 erable individual variation on this point. The average taken 

 from fifteen observations by me is 8.3 weeks. Therefore, the 

 marked increase in the development of the mammary glands of 

 the rat between the eighth and ninth weeks evidently corresponds 

 closely with the age of puberty. Thus the second marked in- 

 crease in the size of the mammary glands is readily accounted 

 for. 



3. Lumen of the ducts 



The time of appearance and method of formation of the lumina 

 of the milk-ducts in various animals have been desribed by vari- 

 ous authors. In the all^ino rat at birth a small irregular slit- 

 like lumen is present in the primary duct (fig. 14). This lumen 

 when traced proximally disappears in the intra-epi dermal por- 

 tion of the primary duct, but when traced distally becomes 

 continuous with the more regular rounded lumen of the second- 

 ary ducts. The tertiary and in fact all of the remaining ducts at 

 birth possess lumina throughout their entire extent. The ter- 

 minal buds also present distinct lumina to within 20 or 30 micra 

 of their distal extremities. The lumen of each bud does not 



