574 THE PHYSIOLOGY OF REPRODUCTION 



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 



FIG. 139. Section of developing mammary gland of horse. (From 

 S chafer, after Hamburger.) 



s, sebaceous glands ; v, blood-vessels. 



the ninth day after conception, on reflecting the skin from the 

 abdomen, the entire surface is found to be covered with a layer 

 of glandular tissue, the margins of the individual glands being 

 practically contiguous, each of them having a diameter of from 

 five to eight centimetres. Sections show that the formation of 

 alveoli (i.e. definite secretory structures) has begun at this 

 period, especially at the periphery, where the gland is generally 

 somewhat thicker than in other parts. 



From this stage onwards the growth of the ducts and the 



