26 AMEBOID MOVEMENT 



The nuclei are small and very numerous and the crystals are 

 well formed and numerous, each in a small vacuole, and of a 

 size about two or three times those found in proteus. It will be 

 seen therefore that there are only small bodies in this ameba, 

 none of which (excepting the contractile vacuole) are large 

 enough to change the course of the endoplasmic stream, and 

 streaming is thus reduced to what might be called a typical condi- 

 tion. 



In this ameba the endoplasmic stream flows uniformly towards 

 the anterior end where it spreads out slightly so as to preserve 

 the same general diameter of the ameba, for it is a characteristic 

 of this ameba that the anterior end is of about the same diameter 

 as the posterior, when in clavate form. The ectoplasmic tube 

 is built at the anterior end, and remains as constructed until it 

 is drawn in at the posterior end to form endoplasm. It is not 

 all the time undergoing changes such as are observed in proteus. 

 This characteristic is very well shown by focusing with the high 

 power of the microscope on the upper surface of the ameba. The 

 immobility of the ectoplasm is much more readily observed in 

 laureata than in perhaps any other species, a condition that is 

 due chiefly to the large crystals whose displacement is the most 

 convenient criterion of ectoplasmic mobility. 



The ectoplasmic tube is not as thick as in proteus, though it 

 appears to be more solid than in that species. It is thrown into 

 folds at the posterior end as it is liquified to form endoplasm, 

 which indicates a firm texture of the ectoplasm. As to the 

 endoplasmic stream, it presents no visible characteristics which 

 set it apart from the fluids of physics; it moves most rapidly 

 in the middle, and gradually less rapidly as the ectoplasm is ap- 

 proached. There is no backward movement of the ectoplasm 

 against the sides of the pseudopod at the anterior end nothing 

 approaching a "fountain current" which indicates that the trans- 

 formation of endoplasm into ectoplasm is rapid and complete. 

 That is, all the endoplasm which reaches the anterior end is turned 

 into ectoplasm. Typically this would result in an ameba of 

 average size, in a layer of ectoplasm of a thickness of about one- 

 seventh of the diameter of the pseudopod (for the area of the 

 cut ectoplasmic tube would equal the area of the endoplasmic 



