202 J. A. MYERS 



moil ill the mesenchyma immediately surrounding the gland. 

 An occasional small blood vessel is seen coursing toward the 

 mammary gland area. 



The mammary gland anlages show about the same stage of 

 development as in the preceding stage. A study of all six pairs 

 shows that the inguinal mammary glands are slightly behind 

 the others in their stage of development. 



Seventeen days. At seventeen days and two hours the emi- 

 nences described in the previous stages have disappeared. The 

 gland areas instead appear as slight depressions or pits on the 

 surface of the skin. These mammary pits represent the point 

 of ingrowth of the epithelium. The epidermis is slightly thicker 

 than in the preceding stage and in the regions of the mammary 

 glands presents a very definite basement membrane. The gland 

 anlages now measure only about 0.05 mm. in length. 



Eighteen days. Fresh preparations, sections, and wax re- 

 constructions from fetuses of eighteen days and nine hours show 

 a definite mammary pit on the surface of the epidermis over each 

 future nipple area (figs. 2 and 8, n.p.). In cross section the 

 stratum germinativum is now depressed so as to form a shallow 

 funnel-shaped outline. The mouth of the funnel is directed 

 toward the surface and is partly filled with epithelial cells which 

 show traces of cornification and desquamation. Intercellular 

 vacuoles are also being formed. The outlet of the funnel ex- 

 tends into the corium and becomes continuous with the anlage 

 of the primary mammary duct. 



At this stage the gland anlage, which in the earlier stages was 

 an oblong, ellipsoidal mass of epithelial cells, has increased in 

 length. Its deep part now becomes the anlage of the primary 

 duct, while its superficial portion is undergoing vacuolization, 

 cornification, and desquamation, thus forming the pit superficial 

 to the primary duct. The end of the primary duct anlage 

 directly beneath the surface pit is attached to the epidermis and 

 throughout this paper will be designated as the attached end. 

 The opposite end of the anlage is unattached and throughout 

 this paper will be known as the free end. The stratum germi- 

 nativum of the adjacent epidermis continues over the mam- 

 mary anlage as its future primary peripheral layer of cells. The 



