70 THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



variations in length and variations in breadth. While the mitochondria in the 

 acinus cells of the pancreas, for instance, vary greatly in length, filamentous and 

 rod-like forms predominating, their girth is remarkably uniform, throughout the 

 whole tissue, in individual cells and in different parts of the same filament. The 

 ends of the filaments are never tapering. They are quite abruptly but smoothly 

 rounded off. Similarly in other tissues the diameter is fixed, but the length is not; 

 one is stamped on the cell through its organization, the other is not. We have 

 here two attributes, independently variable, which may perhaps be influenced in 

 different ways. 



Evidently mitochondria increase in size by growth in length, probably by the 

 addition of material at their extremities. That is to say, when mitochondrial 

 substance is added the extension is always lengthwise, never lateral. It is equally 

 true that when foreign material, like starch, pigment, or fat is deposited within the 

 mitochondria, expansion is always provided for by increase in girth. There 

 must be some difference in the method of addition. At present no explanation is 

 available. 



Much confusion would have been avoided if investigators could have come to 

 reaUze that the same material under different conditions may assume different 

 forms. The unfortunate part about it is that the conditions are almost wholly 

 unknown to us. But whatever they may be, it is altogether likely that they operate 

 in much the same way in plants and animals, because all forms of mitochondria 

 are represented in both. 



We have abundant evidence that mitochondria are semi-fluid in consistency. 

 They flow together and fuse to form large droplets, under certain conditions, and 

 in the streaming protoplasm of plant-cells their form is continually changing in 

 response to currents and eddies in the stream. This semi-fluidity, together with 

 their lipoid-like properties, is responsible for their remarkably smooth and even 

 outUnes and rounded ends. It precludes rough excrescences, sharp angles, and 

 pointed ends. 



The very pronounced property which they have of being able to take in sub- 

 stances from the surrounding cytoplasm and heap them up and concentrate them 

 in their interior is responsible for manj^ of the swellings and enlargements which 

 occur in certain situations in the course of the filaments as well as in the smaller 

 rods and granules. 



It is hard to say how great a part osmotic factors play in molding the shape 

 of mitochondria. The mitochondrial substance is sometimes denser in the periph- 

 eral parts of the filaments and larger granules and bears some semblance to a 

 membrane. The Lewises (1915, p. 373) found that they could alter the form of 

 mitochondria by changing the osmotic pressure of the fluid bathing the cells in 

 their tissue cultures. With hypotonic solutions there was a marked increase in 

 size, with hypertonic a decrease. The osmotic pressure of the blood is very con- 

 stant and has been so, if Macallum's work is correct, for untold ages in evolution. 

 Since the same blood bathes tissues containing entirely' different mitochondria, it 

 seems that the osmotic pressure of the fluid about the cells must play a very minor 



