STUDIES ON THE MAMMARY GLAND 217 



Variation 



Individual variations in the development of the mammary 

 gland are so frequent that at least mention should be made of 

 them. Moreover, no work on the mammary gland should be 

 regarded complete until the conditions have been studied in 

 a sufficient number of individuals to rule out all possibility of 

 error from individual variations. 



Rein ('82) found many individual fluctuations in the develop- 

 ing mammary gland of human. In one pig embryo of 1.5 cm. 

 Schultze ('93) found only the milk line while in another embryo 

 of about the same size he found the 'primitive Zitzen.' Hen- 

 neberg ('00) found in one rat embryo of eleven days no indica- 

 tion of a manmaary streak while in another embryo of the same 

 age a well developed streak appeared only on one side. Rau- 

 bitschek ('04) states that probably no other organ is subject to 

 such great fluctuations in its development as the mammary 

 gland. 



In the present study, it has been noted that in the eighteen 

 day and nine hour stage of the albino rat fetus some of the 

 glands possessed anlages of only the primary ducts while in 

 others there were secondary ducts. Also the lumen began to 

 appear in one individual of this stage while in others there was 

 no trace of a lumen present. The lumen continued to develop 

 until at twenty days and six hours it was represented by a con- 

 siderable cavity in some part of most of the ducts. Yet even at 

 this stage an occasional individual possesses a gland without 

 the slightest manifestation of a lumen. 



The number of mammary glands of the rat likewise is subject 

 to indi-vddual variation. Schickele ('99) found that in 6.66 per 

 cent of the rats examined, only 11 nipples were developing. In 

 80 per cent of his rats 12 nipples (the normal number) were pres- 

 ent. WTiile in 13.33 per cent there were 13 nipples present. In 

 no case did he find more than 13 nipples. Henneberg ('00), 

 Myers ('16), also reported a variable number of glands in albino 

 rats. Schultze ('93) in describing the mammary glands of a rat 

 embryo of 1.2 cm. mentioned only two thoracic pairs of glands 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 22, NO. 2 



