PROTOPLASM 43 



accompanied by two adjacent series of smaller vacuoles, discharges its 

 contents through a definitely localized but temporary ectoplasmic pore, 

 whereupon it disappears completely. Its place is then taken by another 

 formed by the union of secondary adjacent vacuoles (Fig. 19). These 

 latter vacuoles arise in turn from still smaller ones, which seem either to 

 be derived ultimately from vacuoles containing granules that dissolve, 

 or to arise de novo as fluid centers causing the protoplasm to form a 

 membrane by gelation. These observations have an interesting bearing 

 on the problem of the origin of ordinary sap vacuoles (p. 94) In amoebse 

 the membrane of the contractile vacuole appears to be formed anew 

 periodically by gelation (Day, 1927) and shows certain resemblances to 

 the plasma membrane in its permeability (Morita and Chambers, 1929). 



Contractile vacuoles have usually been regarded as excretory organs, 

 but there is evidence that they are mainly concerned in the regulation of 

 hydrostatic pressure. ^^ In Spirogyra, Lloyd (1925, 1928a6) has shown 

 that the action of contractile vacuoles is concerned in the movement of 

 the gametes at the time of conjugation. 



The Ectoplast of Protista. — The ectoplast shows its most elaborate 

 structural differentiations in Protista, where it clearly has several func- 

 tions — protective, motor, excretory, and sensory (see Minchin, 1912, 

 Chapter V). In many forms there is a relatively tough outer envelope, 

 or "pellicle," in addition to the more fluid hyaline ectoplasm. The 

 origin and the degree of development of this envelope, however, are not 

 the same in all cases. Commonly, it seems to arise as a modification 

 of the outer region of the ectoplast. In Amoeba proteus, Mast (1926) finds 

 the internal protoplasm ("plasmasol") to be surrounded by a fairly rigid 

 granular layer ("plasmagel") and this in turn by a tough "plasmalemma" 

 about 0.25m thick. The "periplast" of flagellates seems to represent the 

 entire ectoplast modified. In still other forms the resistant envelope is 

 formed indirectly by secretion. 



Among the ectoplasmic structures with a motor function the simplest 

 are the pseudopodia; in the larger ones there is a core of endoplasm, but 

 the more delicate "filose" ones consist entirely of ectoplasm. Minute 

 suspended particles can be detected in hyaline pseudopodia by the use of 

 dark-field illumination, their Brownian movement affording an index of 

 the fluidity of the ectoplasm (Bayliss, 1920). The cilia on the velum in 

 certain molluscan larvae have the form of thin blades, and each of these 

 is composed of a number of long plates (Fig. 20, C). In Spirillum long 

 contractile filaments are separable, but in Chromatium they are firmly 

 united (Metzner, 1920). Cilia, which are short and numerous and show 

 rhythmic pulsation, ^^ cirri, which are formed of tufts of cilia, memhranellce, 



"" See Calkins (1926); also Weatherby (1927) and Day (1930). 

 ^* For discussions of the structure and mechanics of flagella and cilia, see Heiden- 

 hain (1911), Lundeg&rdh (1922), Metzner (1920), Gray (1928), and Petersen (1929). 



