178 The Structure of Protoplasm 



hyaline zones they become mixed and the granules reach the 

 periphery. 



Tubes not only contract and relax, but they shorten and lengthen, 

 bend and straighten, and undergo various contortions which indi- 

 cate that there is some sort of a contractile mechanism. This is 

 undoubtedly the gel layer. Tubes usually show a considerable 

 amount of shortening during the forward flow and the posterior end 

 is carried forward rather rapidly. During the backward flow they 

 lengthen but do not regain their former length. Sometimes tubes 

 contract locally and completely stop the streaming of the endoplasm 

 through the constricted region for a short time. The endoplasm 

 usually then streams in both directions from the constriction. Such 

 tubes usually relax and the flow of the endoplasm is resumed through 

 the tube. Sometimes there is a permanent obliteration of the flow 

 and the tube later disappears. Other tubes undergo a more or less 

 permanent increase in size. Such changes and alterations are corre- 

 lated with shifting increases and decreases of the advancing ends. 

 I have noted, occasionally, small pieces a few millimeters in size, on 

 the bottom of a Petri dish partly filled with water, send out tube-like 

 elongations at various angles which slowly squirm like a bunch 

 of worms. 



We come now to the transition zone which spreads out fanlike 

 from the anterior end of the tube to the broad crescentic lobulated 

 anterior end. It is a difficult region to understand and to present. 

 There is nothing in the ameba which corresponds to it. One should 

 bear in mind that the slime mold moves forward by a tearing down 

 at the posterior end and building up at the anterior end with the 

 protoplasm which is transported in the endoplasm from the posterior 

 end and other retracting lateral ends and areas. It is obvious that 

 the plasmolet cannot continue to advance or build up in all regions 

 of the crescentic periphery if it is to move forward. One sector 

 builds up more rapidly than adjoining ones. The latter sooner or 

 later retract by contraction and solation and transport of the proto- 

 plasm to the more rapidly spreading or advancing sector. Corres- 

 ponding with this, there is a progressive enlargement and building 

 forward of the channels leading to the advancing sector and a diminu- 

 tion and disappearance of those that supplied the adjoining and 

 retracting ones. During this transformation the broad fan-shaped 

 plasmodial sheet undergoes transformation. The thick anterior part 

 becomes thin and a new thick part is built up in front in the active 

 sector. The middle part of the sheet develops thin areas between the 



