AMEBOID MOVEMENT 21 



certain number of longitudinal canals or grooves by dense walls, 

 which are of a temporary nature, being broken down and built 

 up from time to time. It is easy to distinguish one canal from the 

 other in this species, the currents being at first more or less 

 parallel, but terminating at the forward end, by their coalescence, 

 as a single mass of liquid (p. 63)." But Penard questions Leidy's 

 conclusion that the walls are of ectoplasm : "Moreover Leidy de- 

 ceives himself without any doubt in considering these partitions 

 as folds of the ectosarc. The latter, in the rhizopods, is not a 

 special substance, it is a plasma of surface, specialized for the 

 functions which it has to perform, capable of modification as to 

 its intimate structure, but only so temporarily (p. 63)." 



Although it is a very simple matter to prove to one's satisfac- 

 tion the mere existence of these folds a few minutes' observation 

 under the high power of the microscope will do that it is a much 

 more difficult matter to observe how these folds originate, because 

 of the incessant changes going on, as recorded by Leidy. 



Very young or small pseudopods in protcus have the same 

 general appearance as the pseudopods of other large species 

 (dubia, laureata, discoides, annulata, etc.) ; that is, there is a cen- 

 tral axial stream of endoplasm surrounded by a layer of ectoplasm. 

 But there is one difference even here, and that is the greater 

 thickness of the ectoplasmic walls in proteus in proportion to the 

 diameter of the pseudopod. The ectoplasmic tube however is not 

 solid throughout, but is more or less honeycombed, somewhat 

 like a network, with the spaces filled by endoplasm. 



If the ectoplasm is actually endoplasm that has passed into the 

 gel state, then the honeycomb condition just described resembles 

 an intermediate stage where only a part of the endoplasm has been 

 transformed. This network of endoplasm is strong enough how- 

 ever to impede the flow of the main stream of endoplasm along 

 the sides of the pseudopod; but when large objects, such as the 

 nucleus or food masses, too large to be readily carried in the endo- 

 plasmic stream, impinge against the imperfectly solidified sides of 

 the tube of ectoplasm, the innermost strands of the spongy net- 

 work of ectoplasm snap, usually with readiness, allowing the large 

 object to pass by. 



The surface of a young pseudopod is smooth, a cross section 



