420 J. A. MYERS 
but only a few have made a detailed study of this growth. Lan- 
ger (751) called attention to the growth of ducts at this period 
in human. Ancel and Bouin (’10 and ’11) showed that during 
the period of puberty the mammary gland in the rabbit reaches 
a diameter of 2 to 38 cm. Frank and Unger (’11) showed that a 
very decided growth occurs in the mammary glands of rabbits 
during puberty. Schil (12), in a careful series of observations 
on rabbits, found that the glands gradually increase throughout 
the first period of heat, after which they undergo slight regressive 
changes until the next period of heat. If pregnancy does not 
occur, the glands proliferate with each heat. The regression 
following heat depends upon the interval between two periods. 
Brouha (’05) has shown in Vespertilio murinus, in which there 
appears a single ripe follicle each year, that between two preg- 
nancy periods the gland may return to the infantile stage. The © 
fact that the glands of some rats at ten weeks of age show less 
development than those in others at nine weeks may be due to 
slight regressive changes since the first ovulation, as this ovu- 
lation may occur earlier than nine or ten weeks (Lantz, 710; 
Jackson, 712). However, owing to the fact that careful studies 
on the oestrus cycle in the albino rat have not yet been published, 
the question as to changes in the mammary gland between and 
during successive periods of heat has been omitted in this study. 
The present study, my previous work (Myers, ’16), and the 
findings of the above-mentioned authors lead to the conclusion 
that there is in the female of several animal forms a very marked 
development of the mammary gland at the time of puberty. 
Since such development does not ordinarily occur in the glands 
of the male or in castrated females, it seems probable that these 
changes are at least stimulated by simultaneous changes in the 
ovaries. 
The time and manner of formation of the lumina of the milk- 
ducts were previously discussed (Myers, ’17 a). Suffice it to 
say here that the method of formation of the lumen already 
begun in the fetus is continued in the postnatal stages until 
the lumina are completely developed. The lumen of the intra- 
epidermal part of the primary duct forms an exception, however, 
as it develops by the process of degeneration and desquamation. 
