50Q DIFFERENTIATION OF CELLS. 



is composed of the two preceding elements, the yellow and white fibrous, 

 interwoven with each other so as to constitute a porous structure, with a 

 multitude of intercommunicating spaces. It is to be understood that 

 these interstices are wholly distinct from cells; hence the inapplicability 

 of the term cellular, sometimes employed for this tissue. Areolar tissue 

 is employed for uniting the various animal parts. Its interspaces are 

 filled with a fluid, which, when in jexcess, is spoken of as dropsical effu- 

 sion. Air, artificially or accidentally introduced at any point into it, may 

 pass to every part, as is illustrated in cases of emphysema. The speci- 

 men from which the figure is taken was in this manner inflated. 



By the differentiation of cells is meant the assumption of a variation 

 Differentiation in their structure from which follows, as a consequence, the 

 of cells. capacity of discharging new functions. When the red snow- 



alga multiplies, as previously described, each of the young cells resem- 

 bles that from which it was derived in structure, and discharges a simi- 

 lar office. In such a case there is development, but not differentiation. 

 When, on the contrary, a lichen grows on a rock, though the original 

 tendency in development may have been for the production of cells from 

 the first germ absolutely similar in all directions, yet the circumstances 

 of growth are such that very soon the physical conditions under which 

 the cells of different parts of the growing mass are generated become dif- 

 ferent. Those which are next to the rock are screened by the superin- 

 cumbent ones from the sunlight and the air ; they are therefore develop- 

 ed in a comparative obscurity, and in the presence of moisture holding in 

 Acquisition of solution inorganic salts. Under such circumstances, it is to 

 new functions, "foe expected that a modification will ensue in their construc- 

 tion, and that they will be different from those which are developing on 

 the exterior in contact with the dry air ; and, since a change of structure 

 invariably implies a change of function, we might expect, as in reality is 

 the case, that the outer cells are for the obtaining of carbon from the air, 

 being acted upon by the sunlight, and the under cells for procuring moist- 

 ure and such saline substances as may be wanted from the rock surface 

 below. In such a case as this there is a differentiation both of structure 

 and of function. 



Structural differentiation is to be received as the cause of functional 

 Differentiation differentiation, which is its consequence. The former, in 

 in a regular se- every instance, arises from the changed circumstances under 



quenceappears J . \. i-t^- i c - 



to be determ- which cells are being generated, and if this change of circum- 

 med by law. stances follows a regular order or sequence, the differentia- 

 tion will assume the appearance of being guided by a fixed law. Many 

 physiologists, who have not been disposed to accord to physical agents a 

 due influence in this respect, have therefore imputed to the developing 

 cell a power or property of spontaneously pursuing a determinate career. 



