THE SKIN AND ITS APPENDAGES. 75 



The Female Breast. — The mamma of the female is of cutaneous 

 origin, and in its earlier stages of development resembles, and 

 probably corresponds to, a collection of sweat glands arising from 

 a small circular depressed area of skin (Fig. 57 C). The manner 

 of its development is the key to its anatomy. The adult female 

 breast is composed of two elements : 



(a) Glandular tissue derived from the epiblast by a process of 

 inbudding ; 



(6) An intricate arrangement of connective tissue derived from 

 the mesoblastic subcutaneous tissue over the pectoralis major. 



I. Origin of the Glandular Tissue. 



(1) The Mammary Line is a slight ridge of epiblast which 

 stretches along the ventral aspect of the body on each side, from 

 the axilla to the groin, and is the first stage of mammary 

 development in mammalian embryoes. In the sow, for instance, 

 mammae are produced along the whole length of the mammary 

 line. Although this stage has not been seen in the human 

 embryo, it probably does occur, for in 5°/ of bodies a more or less 

 distinct trace of a supernumerary mamma or nipple is to be 

 found, and these occur for the greater part in the inguino-axillary 

 line. Such as occasionally occur on the back or thigh are pro- 

 bably of the nature of dermoid tumours. Supernumerary nipples 

 occur much more frequently in men than in women. This one 

 may expect because the more vestigial the condition of an organ, 

 the greater is the tendency to the production of ancestral 

 (atavistic) forms. 



Developmental Stages. — Seven stages may be recognised in 

 the developmental history of the glandular mammary tissue. 



Four of these take place before birth : 



1st (Fig. 57 A). The deeper layer of epiblast thickens over the 

 mammary area ; this thickening represents a part of the mam- 

 mary ridge or line. This stage is seen in the 2nd month. 



2nd (Fig. 57 B). The thickening becomes depressed, thus giving 

 rise to a slight pit on the surface. 



3rd (Fig. 57 C). From the depression arises a number of buds, 

 exactly similar to those of sweat gland (5 th month). The stalks 

 of these buds form the epithelial lining of the lactiferous ducts. 



4th (Fig. 57 D). The lobular buds, for each bud develops into 

 a lobe, subdivide at their growing extremities. At first solid, 



