176 The Structure of Protoplasyn 



and begins after the contraction and forward flow of the endoplasm 

 has started. 



Solation at the posterior end during the contractile phase is 

 accompanied by the flow of great numbers of granular vacuoles into 

 the endoplasm from the inner wall of the gel layer where they pre- 

 viously had been at rest. Sometimes one can also see many tiny 

 streamlets of granular vacuoles flowing, often in single file, through 

 most of the thickness of the gel layer into the endoplasm. It is 

 evident that solation of the hyaline gel matrix in which the granular 

 vacuoles are embedded occurs during contraction. This is an 

 interesting relationship and is in marked contrast to what occurs 

 at the posterior end when the endoplasmic streaming is reversed. The 

 backward flow of the endoplasm expands and lengthens the posterior 

 end, but it does not usually lengthen to the original length that it 

 had at the beginning of the cycle if the plasmolets are moving for- 

 ward. There is a net loss at the posterior end during each cycle. 

 As the posterior end lengthens and expands, some of the endoplasm 

 comes to rest on the inner surface of the wall and seems to gel in 

 situ. The expansion at the posterior end is presumably due to the 

 internal pressure of the turgid system on the somewhat weakened 

 gel layer resulting from excessive solation during contraction. 

 Expansion and gelation appear to go on at the posterior end until 

 the gel layer becomes thick or viscous enough to automatically 

 exert a sufficiently increased tension to contract and overcome the 

 internal pressure of the endoplasm and reverse the flow. This is 

 undoubtedly correlated with tension changes at the anterior end 

 and in the whole system of tubes and channels. It is quite possible 

 that the weakened condition at the posterior end is in part due 

 to a general decrease in the viscosity of the gel layer throughout 

 most or all of its thickness. 



The tubes consist of a thick gel wall through which the endo- 

 plasm flows back and forth. They are relatively permanent, but 

 still only temporary, structures. The length of time they persist 

 depends upon how rapidly the plasmolet is moving and changing 

 form. By the time a plasmolet has moved forward a distance equal 

 to its length, it has been completely torn down and rebuilt anew from 

 end to end. The posterior end tubes are the oldest part of the Plas- 

 modium. The condition of the slime mold at the time the preparations 

 are made seems to make considerable difference in the general 

 appearance of the gel layer of the tube. In some preparations it is 

 very much lobulated and very uneven; in others it is rather smooth. 



