THE DEVELOPMENT OF THE INTEGUMENTARY SYSTEM. 



451 



mostly of connective tissue and fat, with scattered groups of duct-like tubules. 

 During pregnancy the tubules give rise to the acini by a process of evagination, 

 the cells increasing in number by mitosis. Toward the end of pregnancy each 

 excretory duct and its smaller ducts and acini form a distinct lobe with a rela- 

 tively small amount of connective tissue. The epithelium is low or cuboidal, 

 and fat- begins to accumulate, in the seventh or eighth month, as droplets in the 

 basal parts of the cells. The droplets increase in number and in size, approach- 

 ing the inner end of the cell, until finally the cell is practically filled. At the 

 beginning of lactation the fat escapes into the lumen of the acinus, leaving a bit 

 of ragged cytoplasm with a nucleus. This regenerates into a cell capable of 



Stroma 

 Fig. 395. — Vertical section of the anlage of the mammary gland of a human fcetus of 25 cm. Nagel. 



further activity; and it is probable that the same cell may become filled with 

 fat and discharge its contents several times during lactation. 



During pregnancy and lactation the acini also contain leucocytes which have 

 wandered through the epithelium from the surrounding tissue. These contain 

 fat droplets and are known as colostrum corpuscles. 



At the end of lactation the acini atrophy and disappear, the lobules becoming 

 masses of connective tissue and fat, which contain groups of duct-like tubules 

 and which are so closely joined with one another that they are indistinguishable 

 as lobules. 



Anomalies. 



Anomalies of the Skin. — The epidermis may develop to an abnormal de- 

 gree over the entire surface of the body, forming a horny layer which is broken 

 only where the skin is folded by the movement of the members of the body — 

 a condition known as hyperkeratosis. Or the abnormal development may give 

 rise to irregular patches of thick epithelium — ichthyosis. In either case, hairs 

 and sebaceous glands arc usually absent over the affected areas. 



