6o6 THE PHYSIOLOGY OF REPRODUCTION 



In the virgin animal of about eight to twelve months old 

 mammary tissue cannot ordinarily be detected with the naked eye, 

 but in stained preparations of the connective tissue surrounding 

 "the nipple, it is possible to see the ducts which comprise the gland. 

 These are generally restricted to an area of not more than one 

 centimetre broad. Sections show that the gland at this stage 

 consists entirely of ducts which are lined with a single layer of 

 flattened epithelium, and end blindly. No traces of alveoli are to be 

 seen in the gland. 



By the fifth day after conception a marked change has taken 



Fig. 166. — Section of developing mammary gland of horse. 

 Sharpey Schafer, after Hamburger.) 



«, Sebaceous glands ; v, blood-vessels. 



(From 



place in the gland, which now appears, on reflecting the skin from 

 the abdomen, as a clearly differentiated pink area, circular in shape, 

 and surrounding the position of each nipple. The diameter of this 

 area is from about two to three centimetres. Sections through the 

 gland show that it still consists entirely of ducts, but that these are 

 in a state of active proliferation. The epithelial lining no longer 

 consists of a single cellular layer, but is two or three cells deep, 

 while the individual cells are more swollen than those of the virgin 

 gland, and mitotic figures are commonly seen. 



The mammary gland now grows rapidly, so that on about the 

 ninth Aa,j after conception, on reflecting the skin from the abdomen. 



