404 J. A. MYERS 
milk-pore. As the primary duct is traced toward the tela sub- 
cutanea a distinct lumen appears and the wall is seen to be com- 
posed of two layers of cells which are similar in shape and arrange- 
ment to those of the walls of ducts of corresponding thickness in 
the female. The remaining ducts in the male resemble in number, 
distribution and structure the corresponding ducts in the female. 
The secretion in the lumina appears the same in every respect as 
in the female except that it appears present in slightly larger 
quantities in the male. 
In both sexes the free ends of the milk-ducts of the abdominal 
and inguinal glands are imbedded in the medial margin of the 
inguinal fat pad, but in most cases they have not penetrated this 
pad very deeply. Besides the ducts growing toward the inguinal 
fat pad, the pad itself has increased in size. 
Two weeks 
It has been pointed out by Myers (’16) that the nipple (in the 
female rat) takes on a very rapid development during the second 
week. The present study shows that the greater part of that 
development occurs in the latter part of the second week. The 
nipple becomes so large that it is very conspicuous to the naked 
eye in living animals and has been used to determine the sex at 
this age (Jackson, 712). Microscopic sections and wax recon- 
structions (fig. 13) show that the nipple is much more elevated 
than in. the preceding stage. It is at this time a somewhat 
elongated structure and has not yet taken on the typical form of a 
nipple. The shailow groove which was present around a part of 
the base of the nipple in earlier stages is now somewhat deeper. 
This groove now entirely surrounds the base of the first thoracic 
nipple, except on the caudal side where the base of the nipple 
becomes continuous with the surface of the surrounding skin 
withouta groove. Incaseofthesecond and third thoracic glands, 
the abdominal gland, and the first inguinal gland, the groove 
extends around the base of the nipple except on the lateral side. 
In the second inguinal gland the groove is absent on the cephalic 
side. From the bottom of this groove, which lies immediately 
