Development of the Mammal 321 



As was said above this differentiation begins 

 at about the tenth week and is usually com- 

 pleted by the end of the third month. Ab- 

 normalities in the development of these struc- 

 tures will be noted below. 



DEVELOPMENT OF THE MAMMARY GLANDS 



The mammary glands, being so typical of 

 the mammalia, should, perhaps, be mentioned 

 here, though their development, like that of 

 the hair, another mammalian character, may 

 be found in almost any text-book of histology. 



Among most of the lower mammalia the 

 mammary glands are first seen, in early em- 

 bryos, as two lines of thickened epidermis, the 

 milk ridges, one on either side of the abdominal 

 wall. These ridges become more prominent 

 at certain places where the glands develop, 

 while they disappear in the intermediate re- 

 gions. Similar milk ridges are said to occur 

 in some early human embryos, but it is likely 

 that the human mammary gland normally 

 begins as a single circular thickening of the 

 epidermis, which grows downwards, as a spher- 

 ical mass, into the dermis. This mass loses 

 its spherical form by the outgrowth of lobes 

 into the surrounding tissue ; these lobes con- 



