STUDIES ON THE MAMMARY GLAND 415 
lieved the depression or pocket to be like the nipple pocket of 
marsupials. Rein (’82) studied the gland in a few of its stages 
in the white mouse and white rat. He concluded that the pocket 
surrounding the nipple in the adult has nothing to do with the 
nipple pocket of marsupials, but that it is identical with the 
marsupial pouch. Bresslau (’02, ’10, and ’12), in a series of 
studies, finally concluded that the sheath or pocket surrounding 
the nipple of the adult rodent and insectivorous mammals de- 
velops quite independently of the nipple pocket of marsupials 
or the marsupial pouch. The groove or sulcus at the base of 
the nipple in the albino rat was mentioned in my earlier work 
(Myers, 716), but its formation was not discussed at that time. 
The shallow groove which appears around the nipple in the new- 
born (fig. 1) is undoubtedly a remnant of the prenatal mammary 
pit which the growing nipple has not yet completely occupied. 
Sections through nipples of twelve hours, one day, two and three 
days after birth show that the sulcus around the new-born nipple 
has disappeared; in other words, the nipple completely fills the 
mammary pit. The very slight depression that surrounds only 
a portion of the base of the nipple at five days and continues to 
deepen and become more extensive until it forms a pocket con- 
taining nearly the basal half of the nipple at the tenth week 
apparently develops independently. We must therefore con- 
clude that the pocket in which the base of the nipple of the adult 
virgin mouse and rat rests is not, as Gegenbaur believed, homol- 
ogous with the nipple pocket of marsupials, but, as Bresslau 
suggested, it is an entirely independent structure which in the 
albino rat at least develops after birth. 
The epithelial projection or hood was described in Mus decu- 
manus and Mus musculus by Gegenbaur (’76). Later Rein 
(82) observed such projections in white mice and white rats 
and described them.from sections of fetuses as two solid epi- 
dermal thickenings which surround the mammary anlage. He 
described them as being somewhat curved with concavities di- 
rected toward each other. Rein found similar structures present 
in the mole. He believed that they have to do with the for- 
mation of the pocket around the nipple. Klaatsch (84) also 
