STUDIES ON THE MAMMARY GLAND 355 



region of the abdominal and inguinal glands do not add many 

 difficulties in the technique. 



Sections for microscopic study were prepared by fixation in 

 Zenker's fluid, and embedding in paraffin. The sections were 

 cut 10 micra thick, mounted serially and stained with Mallory's 

 connective tissue stain or with alum-hematoxylin. 



The albino rats used were in good health. After weaning, at 

 the age of three weeks, they were fed upon whole wheat (Gra- 

 ham) bread, soaked in whole milk. They were in general of 

 average weight or above, as indicated by the following gross body 

 weights of the rats from which the specimens represented in the 

 figures were obtained: newborn, about 4.5 grams; 1 week, 8.5 

 grams; 2 weeks, 15.5 grams; 3 weeks, 30 grams; 4 weeks, 53 grams; 

 5 weeks, 54 grams; 7 weeks, 75 grams; 9 weeks, 114.5 grams. 



OBSERVATIONS 



1. General arrangement of glands and ducts 



As shown in figure 1, the mammary glands of the albino rat 

 may be grouped according to the regions which they occupy. 

 The three most cephalic pairs of glands lie in the thoracic region, 

 hence are designated thoracic mammary glands. Passing cau- 

 dad, the next pair lies in the abdominal region at about the 

 level of the umbilicus. Henneberg ('00) speaks of this pair as 

 the abdominal mammary glands. The two remaining pairs, 

 known as the inguinal mammary glands, lie in the inguinal 

 region. 



Each of the first pair of thoracic glands lies slightly cephalad 

 to, or at the level of, the fore limb. Each adult nipple is located 

 about 10 to 12 mm. from the mid-line. In the case of the sec- 

 ond pair of thoracic gland as a nipple is found immediately behind 

 each forelimb. Here the distance from the nipple to the mid- 

 line is somewhat greater, being about 15 to 16 nun. The dis- 

 tance between this and the first pair of thoracic glands is nor- 

 mally about 30 mm. The last pair of thoracic glands is veiy 

 closely associated with the second pair, the nipples being only 

 about 12 to 15 mm. apart. The distance from each nipple to 



