DEVELOPMENTAL MECHANICS. 155 



and through which many formations are attained. The effect 

 of increased blood-supply on the increase of connective tissue in 

 the affected parts is another instance. 



These complex components seem relatively simple in com- 

 parison with others which must be postulated before we can 

 begin the analysis of many structures. 



As an example of these the following may be formulated, if 

 only provisionally ; for if we never have the courage to begin 

 we shall never escape from our ignorance. 



The cells of all tubular and acinous glands have a bipolar 

 differentiation ; they have a basal surface which serves to take 

 up nutriment from the adjoining capillaries, and opposite this 

 a secreting surface ; both surfaces are separated by the whole 

 diameter of the cell ; the remaining surfaces are merely sur- 

 faces of contact with the neighboring cells. Metabolism is 

 carried on in the direction of the axis uniting the polar sur- 

 faces, which direction is usually that of the greatest dimension. 



The arrangement of the cells in lobules in the fully developed 

 mammalian liver, which is a reticular gland with the narrowest 

 possible meshes, viz., meshes only the breadth of a single cell 

 in diameter, causes the cells to be multipolar in the above 

 sense, for each cell has several nutriment-absorbing and several 

 secreting surfaces. The secreting and nutriment-absorbing 

 surfaces are removed from one another by only half the cell- 

 diameter. The lobular structure composed of these cells repre- 

 sents, so far as its form is concerned, merely a cast of the 

 interstices between the meshes of the network of tubular blood 

 capillaries. 



Inasmuch as the lobular structure, molded as it is on the 

 blood capillaries, presupposes the small-meshed reticular type 

 and this in turn the multipolarity of the liver cells, we may 

 regard as the primitive factor in all these deviations from the 

 tubular type of other glands, the change in the polarity of the 

 liver cells, and we may say accordingly : The transformation of 

 the tubular type, which is also present at first in the mam- 

 malian liver, into the definitive lobular type is the consequence 

 of the differential change of the original bipolar nature of the 

 liver cells to a multipolar nature, or ; the multipolar diffcrenti- 



