234 



NORMA L HISTOLOG Y. 



cells lining its acini produce drops of fat in the cytoplasm bor- 

 dering on the lumen, and these are subsequently discharged into 

 the lumen, forming the fat or cream of the milk. The casein of 

 the milk appears to be produced in the following manner : it has 

 been observed that during lactation the nuclei of some of the cells 

 present changes in form that lead to the inference that they undergo 

 division by the direct mode — i. e., without passing through the 

 phases of karyokinesis. It thus happens that some of the epi- 

 thelial cells contain two nuclei. These cells, after a while, project 

 into the lumen of the acinus, the two nuclei lying in a line perpen- 



Fig. 210. 



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O 



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o 



Fig. 209. — Dividing epithelial cells from the mammary gland of the guinea-pig. (Michaelis.) 

 The figure represents the proliferation of the cells by the indirect mode before lactation 

 has been established — i. e., during the maturation of the gland. 



Fig. 210. — Colostrum-corpuscles and leucocytes from the colostrum of a guinea-pig. 

 (Michaelis.) 



dicular to its wall (Fig. 211). It is supposed that the nuclei nearest 

 the lumen become detached, together with some of the cytoplasm, 

 and that the chemical constituents of the nucleus and cytoplasm enter 

 into the formation of the casein. Such free nuclei have been observed 

 in the lumina of the acini, and it is known that the chromatin which 

 they contain disintegrates and eventually disappears (chromolysis), 

 so that it is not found in the secreted milk. Whether this is, 

 in detail, the exact process by which casein is formed is still an 

 open question, but that there is an increase in the nuclear materials 

 of the secreting cells appears certain, and that these nuclei furnish 

 the nucleic acid entering into combination with proteid substances to 

 form the nucleo-albumin, casein, is most probable. The site of the 

 origin of milk-sugar is not known; it appears to be elaborated 

 within the secreting cells. 



