284 EXPERIMENT STATION RECORD. 



matic substances. In some cases this retrogressive change is apparent 

 before the nucleus leaves the epithelial cell, although much the larger 

 portion are intact when they reach the alveoli. 



The form of the epithelial cells exhibits some very typical changes 

 during lactation. According to Michaelis they are first cubical, tben 

 papilliform, then cubical again, and finally flat. It is quite possible 

 that the albuminoids of milk are produced by these cells at certain 

 stages of the secretion, when they protrude like papillae into the lumen 

 of the alveoli. 



Until recently fat has been regarded as a result of a degeneration of 

 the glandular epithelium. If this proposition were correct it would 

 call for a double layer of epithelium in the secreting glands, while, as 

 shown above, there is only one layer. Consequently the formation of 

 fat in the mammary glands is not due to a degeneration of the glandular 

 epithelium, as in the case of the sebaceous glands, but rather to an 

 infiltration of fat which the epithelial cells extract from the circulating 

 supply of blood and lymph. The epithelium secretes or separates the 

 fat itself by extracting it out of the fluids (soaps) furnishing it, and no 

 breaking down of the epithelium takes place. 



Regarding the disputed question as to whether the fat globules are 

 inclosed in a membrane, Michaelis concludes that they are not. Not 

 the slightest trace of a membrane was found. 



As milk sugar is not a morphological element of milk, it is considered 

 impossible to determine its origin until further advancement has been 

 made in micro-chemistry. 



The casein, however, is evidently composed of albuminoid bodies 

 secreted by the cells, and formed by disintegration of the nuclei. The 

 latter process accounts for the nuclein content of milk shown by Heiden- 

 hain and Nissen. 



Since, as shown above, only a few nuclei of the epithelial cells are 

 broken down during the colostrum period, the chemical difference 

 between milk and colostrum is explained ; the milk contains more casein 

 (including nuclein) and the colostrum more albumin. 



Concerning the behavior of the mammary glands in the involution 

 period, the most noticeable feature is the return to the production of 

 colostrum. "The secretion of a guinea pig at the end of gestation can 

 not be distinguished from that produced the day after the young have 

 been weaned." The eosinophil cells appear again, leucocytes penetrate 

 the epithelium in large quantities and break down in the alveoli o> 

 are transformed into colostrum bodies. This is true, no matter how 

 long a time has elapsed since giving birth. The whole process of 

 reformation of the glands takes place very soon after weaning; the 

 alveoli materially diminish in size, and the interstitial connective tissue 

 materially increases. 



