DEVELOPMENT OF THE GLAND. 455 



cession, the ducts are a later acquisition. This reversal of the 

 order of acquisition of parts is in accordance with the prin- 

 ciple stated by Herbert Spencer, that 'under certain circum- 

 stances the direct mode of development tends to be substituted 

 for the indirect.' ' 



Hints regarding the Mstological study of the mamma. 

 The evolution of the mammary structure progresses paripassu 

 with the development of its functional activity. It is the stim- 

 ulus of pregnancy which determines both. Nevertheless, 

 even during the period of its fullest physiological bloom, i.e., 

 during lactation, variations in the degree of functional activity 

 normally take place. Moreover, the same gland may contain 

 lobules which are comparatively at rest, and others which are 

 at the full height of activity. This should always be borne in 

 mind in interpreting the results of histological inspection of 

 this organ, lest erroneous impressions be conveyed. 



The alveolar epithelial cells will, therefore, not be found 

 alike in the different acini, nor yet even in the same vesicle. 

 We may find cuboidal cells, and cylindrical ones, and flattened 

 corpuscles, and in addition, various transitional forms between 

 these tj^pes. 



The nucleus will appear round, or oval, and about 6-7 p in 

 diameter. Sometimes two nuclei may be found in one cell. 

 The radiating striation observed by Rauber in many cells, has 

 already received iriention. It is a noteworthy fact that the 

 cells themselves contain only a very small proportion of fatty 

 granules, whereas the intra-alveolar lumen is often replete 

 with the same. 



In order, then, to study the histology of the gland at the 

 high-water-mark of its functional activity, animals should be 

 chosen which have either just given birth to their young, 

 or are about to do so. For the normal conditions of the 

 human mamma are rapidly transformed by post-mortem change, 

 if not previously altered in consequence of the disease which 

 caused the death of the individual. The organ may be exam- 

 ined fresh, or else hardened and then cut in sections to be 

 stained and mounted in the ordinary manner. 



