LACTATION 



607 



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 

 formation of alveoli proceed rapidly, so that by the twenty-fifth day 



Fig. 167. — Section of mammary gland (human), showing developing alveoli. 

 (From Sharpey Schafer, after von Ebner.) 



b. Connective tissue ; d, undeveloped alveoli ; d', partially developed 

 alveoli ; g, blood-vessel ; m, portion of duct. 



of pregnancy the whole surface of the abdomen has become covered 

 by mammary gland tissue, which may be half a centimetre thick. 

 This tissue is seen in sections to consist for the most part of alveoli, 

 in the cells of which fat globules are in process of formation. 



From about the ninth day onwards to the twenty-fifth it is usually 

 -possible to squeeze a watery fluid from the nipples. During the last 

 days of pregnancy this fluid assumes the characteristics of normal 

 milk, so that by the time of parturition, which occurs in the rabbit 

 on the thirtieth day after conception, the glands are full of milk. 



The multiparous rabbit differs from the virgin in possessing 

 ready-formed alveoli at the beginning of pregnancy. Consequently 



