STUDIES ON THE MAMMARY GLAND 201 



nearest the developing mammary gland are somewhat elongated 

 and arranged in two or thi-ee very regular layers concentrically 

 placed (fig. 1). Outside of the concentric layers, the condensed 

 mesenchymal cells seem to have no definite arrangement. In 

 the condensed mesenchyma is seen an occasional small blood 

 vessel containing nucleated red blood corpuscles. 



Wax reconstructions (fig. 10) show that the differentiated 

 mass of cells which appears circular in cross section, forms an 

 oblong ellipsoidal body which is attached to the epidermis by a 

 very short, constricted neck (nk). 



Sixteen days. In fetuses of sixteen days and twelve hours the 

 mammary eminences still appear on the surface of the skin as 

 slightly elevated areas which in fresh preparations have a 

 somewhat lighter appearance than the surrounding tissue. 



In microscopic sections the epidermis presents the two distinct 

 layers of cells found in the preceding stage. In addition an 

 intermediate layer of cells has appeared in some parts of the 

 skin. In some places the epidermis is slightly thickened to f©rm 

 hair anlages, but in no case were such anlages observed in the 

 epidermis adjacent to the mammary gland anlages. The so- 

 called basement membrane appears as a homogeneous band im- 

 mediately below the stratum germinativum. Just beneath the 

 basement membrane the mesenchymal cells are densely placed 

 thus forming a fairly definite layer. Immediately beneath this 

 layer the mesenchymal cells are less numerous and apparently 

 have no regular arrangement. Mitotic figures are very com- 



Fig. 1 Drawing of a section through the right second thoracic mammary 

 gland region of an albino rat fetus of fifteen days and nine hours. X 300. 

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

 lucida. b.m., basement membrane; cm., condensed mesenchyma; e.s., eminence 

 (mammary hillock) on surface of skin produced by mammary gland anlage; 

 m., loose, irregularly arranged mesenchyma; m.a., mammary gland anlage; p., 

 periderm; s.g., stratvma germinativum. 



Fig. 2 Drawing of a section through the left second thoracic developing 

 mammary gland of a female albino rat fetus of eighteen days and nine hours. 

 X 300. Zenker's fixation; hematoxylin-eosin stain. Drawn with the aid of a 

 camera lucida. b.m., basement membrane; cm., condensed mesenchyma; m.p.. 

 early appearance of mammary pit; p.d., deep portion of mammary gland anlajj;!' 

 (primary duct); s.m.a., superficial part of mammary anlage, becoming cornificd. 



THE AMERICAN JOURNAL OF ANATO.MV, VOL. 22, NO. 2 



