STUDIES ON THE MAMMARY GLAND 857 



thoracic pair is reached, remains about the same for tlie alxlomi- 

 iial pair, then decreases to the last inguinal pair. 



From figure 1 and the above measurements it will be observed 

 that the abdonhnal pair of glands is more closely associated with 

 the inguinal than with the thoracic glands. This arrangement 

 points toward a localization of the glands in only two regions. 

 So far as the distribution of the mammary glands is concerned, 

 the rat may be regarded as occupying a j^osition between those 

 forms which possess a continuous row of mammary glands on 

 each side from the thoracic through the inguinal regions and those 

 in which the mammary gland is confined to a single region. 



The average number of mammary glands in the albino rat is 

 12 (G pairs), but Henneberg ('00) and Frank and linger ('11) 

 have called attention to the fact that the number varies between 

 10 and 14. Henneberg examined 28 embryos from the age of 

 14 days and 20 hours to 15 days and found in 5 cases that super- 

 numerary mammary hillocks appeared. All were in the pec- 

 toral region. The accessory hillocks, which in all cases appeared 

 smaller than the normal, were located in 4 cases between the 

 second and third normal hillock, and in one case caudal to the 

 third. In the adult animals, however, he found that hypermas- 

 tia rarely occurred, as only one case was reported from 150 obser- 

 vations. In this one case the individual possessed a pair of 

 apparently functional glands just caudad to the last pair of tho- 

 racic glands. In the 150 adult rats only one case of hypomastia 

 was found. In this case the individual lacked the first pair of 

 inguinal glands. 



In the present study, the number of glands present was noted 

 in all animals available. From observations made on 100 indi- 

 viduals ranging in age from 10 days to adults, SO were found to 

 possess the normal number of mammary glands. Only one su- 

 pernumerary gland was observed. It was located just caudad to 

 the third thoracic gland on the left side, the right side presenting 

 the normal number. In 12 cases the second thoracic gland was 

 apparently lacking on the right side only, while in 7 other cases 

 the second thoracic gland was lacking on both sides. 



