210 J. A. MYERS 



safely assume tha,t the process already begun in the eighteen to 

 twenty day fetuses continues until the time of birth, thus pro- 

 ducing the continuous lumina found in newborn rats. 



The lumma at birth have not assumed their definitive form, 

 however. In a later study the details of the further develop- 

 ment of the lumina in the postnatal stages of the albino rat 

 will bo described. 



DISCUSSION AND CONCLUSIONS 



In the following discussion, the nipple, the milk-ducts, the 

 epithelial hood, gland stroma, variation, and cephalocaudal 

 sequence in development will be successively considered. 



The nipple 



A comparison of figures 1 and 7 is sufficient to show that in the 

 albino rat fetus slight eminences occur in the region of the future 

 nipples. These eminences evidently correspond to the mam- 

 mary hillocks described in other forms. The mammary hillocks 

 in the rat fetus (as in other forms) are temporary eminences, 

 each being soon replaced, as has been shown, by a shallow 

 depression, the mammary pit. At the bottom of this pit later 

 occurs a slight elevation representing the nipple anlage. The 

 true nipple reaches only a very rudimentary stage of develop- 

 ment in rat fetuses. The latest stage studied (twenty days 

 and six hours) shows the nipple anlage as a rather slight emi- 

 nence at the bottom of the mammary pit (figs. 3 and 9). 



The phenomena of development in the nipple and the asso- 

 ciated hillock and pit are rendered more intelligible by a com- 

 parison with the conditions found in lower forms. 



The mammary hillocks first appear in rat embryos of fourteen 

 days (Henneberg, '00). The present work shows that they 

 persist through the sixteenth day at which time they are less 

 conspicuous than at the fifteenth day. These hillocks apparently 

 occupy the positions of the future nipples. Because of their 

 positions and resemblance to a nipple, Schultze ('92 and '93) 

 in the pig and other species called them primitive nipples 



