THE STin'CTVRAL COMMON KNTS OF I'ROTOI'LASTS 



23 



consist of both h,yaline ectoplasm and granular cndoplasni. Since most 

 of the visible changes in differentiating cells occur in the cytoplasm, the 

 latter has sometimes been regarded as an organ of differentiation , some- 

 what as the nucleus has been called an organ of heredity. 



At the outer surface of the cytoplasm, whether it is surrounded b}'- a 

 thick cell wall or not, there is a film of ultramicroscopic thinness, the 

 plasma membrane. In case a layer of ectoplasm is present, as in the 

 ameba, the plasma membrane is at its outer boundary. Such a membrane 



Fii;. 11. — Portioii.s of two huiiiuii niu^;cle fil)eis. The myofibrils vuu Icusthwi-^e. 

 Modifications in the associated fibrils at regular intervals are responsible for the transverse 

 striations. {Courtesy of General Biological Supply House, Inc., Chicago.) 



evidently develops by the accumulation of certain protoplasmic constit- 

 uents to the exclusion of others and the arrangement of these constituents 

 into a layer having a special type of structure. It seems to resemble in 

 some measure the surface film on a pond or drop of water, Avhere the 

 molecules have been shown to be arranged regularh'^ and closel}' in a 

 pavement-like layer. The elastic and other physical properties of tliis 

 external membrane have been studied with the aid of tlie micromanipu- 

 lator (page 45). Such studies have shown that when the membrane is 

 torn (if not too greatly) it is quickly renewed by the protoplast. 



The physical and chemical properties of the plasma membrane are 

 largely responsible for the physiological behavior of the protoplast, in 

 particular its interaction with its environment. What substances shall 



