216 J. A. MYERS 



insectivorous mammals) some of whom believed it to be homol- 

 ogous with the marsupial pouch or the nipple pocket of marsup- 

 ials. As to the significance of the epithelial hood in the albino 

 rat I have as yet reached no definite conclusion. 



Gland stroma 



The majority of the investigators have observed and described 

 the mammary gland stroma. In the rabbit and man at the 

 end of the period of 'Knospenbildung,' Rein ('82) found the 

 first appearance of the gland stroma. In the albino rat ac- 

 cording to Henneberg, the mesenchyma deep to the first anlage 

 of the mammary gland is condensed. The present work shows 

 that in the rat fetus at fifteen days the mesench^aual cells lying 

 nearest the mammary gland anlage are elongated and arranged 

 in two or three distinct rows around the anlage. At about 

 seventeen and eighteen days, as the primary duct buds out 

 from the main gland anlage it becomes well surrounded with 

 developing connective tissue cells, which at this stage present 

 long fibrous processes. As many as three or four layers of the 

 cells and their fibers surround each duct, while farther from the 

 ducts the connective tissue cells and fibers are arranged parallel 

 with the surface of the skin. A short time before birth, at 

 twenty days and six hours, the ducts are covered with a sheath 

 of fibrous tissue. The connective tissue external to this sheath 

 is somewhat condensed (fig. 6). The sheath which intimately 

 surrounds each duct corresponds to the part which Berka ('12) 

 described as the mantle layer of young girls and older virgins. 

 The condensed tissue external to the sheath he designated as 

 the true stroma. In the true stroma, blood vessels and nerves 

 are found, but the blood vessels are not as abundant as one might 

 expect. 



The fatty tissue enclosed by the gland stroma, which takes 

 an important part in the later development of the gland, was 

 not observed in the fetal stages. 



