468 EXPERIMENT STATION RECORD. [Vol.40 



There are six pairs of nipples, three in the thoracic and three in the inguinal 

 region. Of 100 female rats studied, 80 had normal arrangement of nipples, 12 

 lacked the right nipple of the second thoracic pair, 7 lacked both of this pair, 

 and one had a fourth thoracic nipple on the left side. A simplified terminology 

 for the mammary ducts la adopted. 



Only one primary duct was found to be present in each gland; it branches 

 and rebranches in an intricate manner by the development of lateral buds. It 

 is not certaain whether dints of the different glands communicate. No true 

 alveoli were observed during the ages covered by this study. Croat activity in 

 growth and branching of ducts occurs in the ninth week, which probably corre- 

 sponds to the age of puberty. A large amount of individual variation was 

 observed in the development of the glands, a matter which must be considered 

 by Investigators in conducting physiological experiments, 



II. The fetal development of tin mammary otand in the female albino rat. — 

 Studies <>f other authors have not dealt with changes in the mammary glands 

 of rats between the sixteenth day itiOD and the time of birth live or six 



days later. The author confines his attention largely tn this period. It was 

 found that mammary pits flrsl make their apparance on the seventeenth day and 

 become more definite later. <>n the twentieth day iii the deepest part 01 • 

 pit and surrounded by a narrow furrow is a small eminence which after birth 

 develops into the nipple. The milk ducts begin as solid curds of. epithelium, pro- 

 jeciing inward. On the eighteenth day branching is noted in the primary milk 

 ducts, ami by the twentieth secondary dints are present in all glands with 

 tertiary and terminal ducts beginning tn appear in the glands of the Inguinal 

 region. At this time the ducts are Burrounded by a sheath of fibrous tissue. 

 The lumina of the ducts are formed by rearrangement of cells, resulting in 

 numerous intercellular spaces that later flow together. 



ill. i comparison <>f tin mammary !/i<tn>i* in male and female albino r<it* 

 from the late i't'it stagt t t<< v> week* of age. Male fetuses of is days show no 

 mammary pits, but there is a slight eminence covered with a thickened epithe- 

 lium. <>n the twentieth day neither pits nor eminences 'n\>v the mammary 



gland areas, hut the ducts come directly ti> the surface. In the postnatal Si 

 Studied no nipples OCCUr mi male individuals. Prom the twentieth day n!" ;_'es- 

 tation until the fifth week i>f postnatal life the milk dints of the male resemble 

 those of the female but thereafter undergo little development The number of 

 mammary glands Is more variable in the male than in the female. 



The relation of age to fertility in the rat. Hmin I'. Kim, (Anal. Kea, 11 

 [1916), No. ~>. />/). 269-2S7, figs. 3). — Breeding records <>f 78 females that gave 

 birth to a total of 3,955 young comprised in 585 litters are the basis of this 

 study. All the females considered lived t<> the are nf at least 16 months. The 



number of litters produced per female varied from :? to 16, the average being 

 7.7. As Indicated by the tables the median was 7 and tin' most frequent 



number, 6. 



The young female rat in good physical condition will generally cast her first 

 litter when three months old. and normally produces a Utter a month for some 

 time thereafter. Complete Cessation of ovulation normally occurs about the 

 eighteenth month of life. The greatest number of litters were cast when the 

 mothers were six or -.oven months old. The number was slightly lower at the 

 younger ages and decreased sharply after this point. T •• of all 



litters was 6.7. The largest one contained 1»". yoUBg. The table shows that 

 the medium size was 7 and the most frequent Size, x . with a secondary mode 

 :it 6. The size W86 greatest when the mothers were four months old. the 

 average then being 7.9. At three months the avert ' The average 



