STUDIES ON THE MAMMARY GLAND 413 
and especially in the last half of this week the nipple takes on a 
rapid development (fig. 13). At this time the nipple has reached 
a height of approximately 0.4mm. From the third to the eighth 
week the nipple increases in size and gradually becomes some- 
what conical in shape (fig. 15). During the eighth and ninth 
weeks, the time when the first ovulation normally occurs, the 
nipple begins to appear in size and form similar to the nipple of 
the adult virgin rat. In some individuals it now closely ap- 
proaches the shape of a cone with a height of 1 mm., while in 
others it has not taken on its definitive conical form (fig. 17). 
It is perhaps due to the fact that the nipple has no function to 
perform in the early life of the individual that it remains in a 
somewhat rudimentary state of development during this period. 
The rapid elevation during the second and third week may be 
closely related with the secretion that has already been produced 
in the milk-ducts. After the second and third weeks there seems 
to be nothing to stimulate a rapid growth of the nipple as it grows 
very gradually until near the time of puberty. As the first 
opportunity for pregnancy or the first ovulation approaches, the 
nipple takes on the shape and size of that of the adult virgin 
rat. This development of the nipp’e is apparently closely cor- 
related with contemporaneous changes in the ovary as macro- 
scopic observations show that Graafian follicles are rapidly 
growing in the early part of this stage. Furthermore, the vas- 
cular supply to the ovary very materially increases as the 
period of puberty is approached. 
Rein (’82) found in the mammary glands of the rabbit two 
months after birth that the nipple presents a slight elevation in 
the form of the head of a pin. This elevation is about 1 mm. 
in height with a basal diameter of 1 to 1.5 mm. In rabbits of 
six months Schil (’12) found the nipples are conical elevations 
1 to 1.5 mm. in height and 4 mm. in diameter at the base. Schil 
also states that the nipple is rarely indicated in the new-born 
human. Herz (’00), however, found that the nipple may or 
may not be elevated in new-born children. 
While measurements on the nipples are lacking for many 
forms of animals, the present work together with general state- 
