DEVELOPMENT Of THE MAMMARY GLAND. 281 



The first rudiment of the gland becomes visible about the 

 third month of intra-uterine life, yet in the newly born child 

 we rarely find anything more than the principal ducts, with 

 some indications of ramification, in the form of two or 

 three club-shaped processes ; and even if these be somewhat 

 more developed, the terminal vesicles are always absent, even 

 in those cases where a fluid is excreted; but numerous and 

 closely approximated dilatations then occur, that give to the 

 gland the appearance of a group of sebaceous follicles. 



The ducts continue to increase up to the period of puberty, 

 though only slowly even in the girl ; but they then undergo 

 rapid development, the condition attained remaining persist- 

 ent in the female, whilst in boys it may undergo regressive 

 metamorphosis. In the adult male, as a rule, only some of the 

 principal ducts are present, with a few slightly subdivided pro- 

 jections ; but cases also occur where the glands, having attained 

 the size of a walnut, contain a ramified system of ducts, present- 

 ing forms similar to those of the girl before the period of puberty. 



True acini, constituting the extremities of an already highly 

 ramified system of ducts, occur only in the sexually mature 

 female. At this period of life the gland usually already 

 exhibits the grape-like acinous structure, though the lobules 

 are small, widely separated from each other, the ducts of 

 narrow calibre, and the vesicles small, and more of a cylindrical 

 than of a clavate form. The contents of the entire system of 

 secretory ducts in the yourg woman consist of collections of 

 cells, which, densely packed, form solid masses at their ex- 

 tremities ; whilst in the ducts, which are already pervious, they 

 form a single layer, lining the inner surface. Finely fibrillated 

 connective tissue enters into the composition of the walls of the 

 larger ducts, but those of the smaller are formed of a layer of 

 hyaline tissue, swelling strongly in water, and separated from the 

 stroma on its outer surface by a series of attenuated fusiform cor- 

 puscles. At the extremities of the ducts this layer appears 

 much thicker, and presents, in transverse sections, broad swollen 

 haloes, or zones, in close approximation to each other by their 

 external sharply defined contours, whilst the internal border 

 exhibits fine inflections, with projecting folds, that are evidently 

 due to the imbibition of fluid by the membrane. 



