204 J. A. MYERS 



into the subjacent corium. The ridge extends entirely around 

 the funnel and is the anlage of the epithelial hood, which was 

 described in the postnatal stages of the albino rat (Myers '16). 



Nineteen days. The funnel-shaped epithelial area correspond- 

 ing to the mammary pit at nineteen days and six hours con- 

 tains some cornified epithelium. This is apparently being cast 

 off by the process of desquamation, thus deepening the mam- 

 mary pit superficial to the attached primary duct. 



The primary mammary ducts have made a rapid growth and 

 present secondary ducts in all glands, while in most glands 

 examined the secondary ducts present tertiary ducts. The two 

 inguinal glands present lumina in about the same stage of de- 

 velopment as was described in the thoracic and abdominal 

 glands in the eighteen day and nine hour stage. The rudi- 

 mentary lumina in all glands are slightly further developed 

 toward the free ends of the ducts but are by no means confined 

 to the free ends. Many of the cells near the developing lumina 

 are undergoing mitotic divisions. There is no pyknosis or other 

 eiddence of cell degeneration. 



The anlage of the epithelial hood is composed of elongated 

 cells of the stratum germinativum, but a second layer of cells deep 

 to the layer described as forming a low ridge in the preceding 

 stage is beginning to appear. The ridge now projects deeper 

 into the subjacent corium. Numerous mitotic figures are seen 

 in the epithelial cells in the region of the free edge of the hood. 



The developing hair follicles have grown more deeply into the 

 corium than those described in a preceding stage. Ordinarily 

 the follicles are located a considerable distance from the mam- 

 mary pits. No follicles were observed in the mammary pits. 



Twenty days. At twenty days and six hours well defined mam- 

 mary pits in the epidermis represent (as in the preceding stage) 

 the regions of the mammary glands. Wax reconstructions, 

 how^ever, show that at the bottom of each pit there is a rounded 

 elevated portion of the epidermis (fig. 9, n.a.). This elevated 

 part is the anlage of the nipple. In the preceding stage as noted 

 the depression or funnel was partly filled with cells, which be- 

 came cornified as age advanced, thus giving the integument 



