STUDIES ON THE MAMMARY GLAND 209 



amined showed such an infiltration of leucocytes as Keiffer ('02) 

 and others have described in the human newborn. 



The processes of the connective tissue cells have elongated and 

 when treated ^\ith Mallory's connective tissue stain many of 

 them now appear as true white fibrous connective tissue fibers. 

 Weigert's elastic tissue stain revealed no trace of elastic fibers 

 at this stage. The developing connective tissue has so differ- 

 entiated that the anlages of two adult parts may now be recog- 

 nized. That part immediately adjacent to the ducts forms a thin 

 sheath around them. This sheath is the anlage of the mantle 

 layer. While the connective tissue between the ducts represents 

 the anlage of the true stroma (fig. 6, m.l., s.t.). 



Lobules have not yet formed in the mammary gland. 



The masses of fat which are so conspicuous in the postnatal 

 stages are not developed at this stage. 



The foregoing stage at twenty days and nine hours brings the 

 description up to the condition at birth which was the starting 

 point in my previous paper (Myers, '16). In newborn rats the 

 lumina were found to extend through the primary ducts (except 

 the intraepidermal portion) into the secondary ducts and to ter- 

 minate in the end-buds. In the primaiy ducts the lumina are 

 small irregular slit-Uke spaces which become continuous with the 

 more regular rounded lumina of the remaining ducts. One can 



Fig. 4 Drawn from a section through the primary duct (near free end) of the 

 left second thoracic gland of a female albino rat fetus of eighteen days and nine 

 hours to show development of lumen. X 550. Zenker's fixation; hematoxylin- 

 eosin stain. Drawn with the aid of a camera lucida. cm., condensed mesen- 

 chymal I.e., small cavities (lacunae) which later fuse to form lumen. 



Fig. 5 Drawing of a tangential section through a secondary duct of the 

 right first inguinal gland of a female albino rat fetus of twenty days and six 

 hours to show developing lumen. X 550. Zenker's fixation; hematoxylin- 

 eosin stain. Drawn with the aid of camera lucida. c, irregularly arranged de- 

 veloping connective tissue cells; c.t., developing connective tissue forming sheath 

 around duct; I., lumen formed by fusion of small cavities (lacunae); I.e., small 

 cavities (lacunae). 



Fig. 6 Drawn from a section through four tertiary ducts of the left first 

 thoracic gland of a female albino rat fetus of twenty days and six hours. X 175. 

 Zenker's fixation; hematoxylin-eosin stain. Drawn with the aid of a camera 

 lucida. t.d., tertiary ducts; m.l., developing connective tissue to form mantle 

 layers; st., developing connective tissue forming true stroma; I.e., lumen. 



