STUDIES ON THE MAMMARY GLAND 197 
as the pectoral portion and the caudal part as the abdominal 
portion. 
In rat embryos of fourteen days Henneberg found that the 
cephalic end of the mammary line has been transformed into a 
structure about the shape of a biconvex lens. This is the earliest 
appearance of the first pectoral mammary hillock. In other 
embryos of the same age the second and third pectoral and the 
abdominal hillocks are beginning to appear. The greater con- 
vexity of each hillock lies embedded in the mesenchyma. The 
remaining parts of the mammary streak and line represented 
by the space between the hillocks are beginning to atrophy. 
At this stage the mammary line for the inguinal glands resembles 
in structure the line for the pectoral and abdominal glands in the 
thirteen day and fourteen hour stage. 
Henneberg found in fifteen day rat embryos that the mammary 
gland anlages are no longer elevated above the surface but that 
their deep surfaces have pressed deeper into the mesenchyma 
thus presenting the 'mammary point' stage. At this stage the 
inguinal glands are still somewhat retarded in their development. 
At sixteen days Henneberg states that the mammary gland 
anlages correspond to the club-shaped stage which Rein ('82) 
found in rabbit embryos. Henneberg did not investigate the 
later stages in the rat. 
MATERIAL AND TECHNIQUE 
The fetuses for the present work were collected in the follow- 
ing manner. Adult males and females were placed in the same 
cage from six o'clock in the evening until six o'clock in the 
morning. As found by Danforth ('16) in case of mice, better 
results were obtained when the females were placed in the cage 
which the males occupy permanently. The females were then 
returned to their respective cages. In all cases of pregnancy 
semination was dated at the ninth hour after the females were 
placed in the cages with the males. The possibility of error in 
the age of the fetuses is plainly obvious. However, the error 
could only be a matter of a few hours. Sobotta and Burck- 
hard ('11) estimated that spermatozoa of the albino rat do not 
