434 



COMPARATIVE PHYSIOLOGY. 



latter is seen t6 arise in the former as very small globules 

 which run together more and more till they may wholly re- 

 place the original protoplasm. 



The history of the mammary gland is, perhaps, still more 

 instructive. In this case, the appearance of the cells during 

 lactation and at other periods is entirely different. Fat may be 

 seen to arise within these cells and be extruded, perhaps in the 

 same way as an Amceba gets rid of the waste of its food. So 

 far as the animal is concerned, milk is an excretion in a limited 

 sense. 



It is, in the nature of the case, impossible to follow with 

 the eye the formation and separation of milk-sugar, casein, etc. 



Fie. 325.— I. AcinnB from mamma of a bitch when inactive (after Heidenhain). II. 

 During secretion of millc. a, b, millt-globules; c, d, e, colostrum-corpuscles; /, 

 pale cells. ' 



But the whole process is plainly the work of the cells, and ia 

 no mechanical sense a mere deposition of fat, etc., from the 

 blood ; and the same view applies to the construction of fat by 

 connective (adipose) tissue. 



Whether fat, as such, or fatty acid, is dealt with without 

 being built up into the protoplasm of the cell, is not known ; 

 but, taking all the facts into the account, and considering the 

 behavior of cells generally, it seems most natural to regard the 

 construction of fat as a sort of secretion or excretion. To sup- 

 pose that a living cell acts upon material in the blood as a 

 workman in a factory on his raw material, or even as a chemist 

 does in the laboratory, seems to be too crude a conception of 

 vital processes. Until it can be rendered very much clearer 

 than at present, it is not safe to assume that their chemistry is 

 our chemistry, or their methods our methods. It may be so ; 

 but let us not, in our desire for simple explanations or undue 

 haste to get some sort of theory that apparently fits into our 



