THE PL ASM ATI C MEMBRANES 109 



& 



especially water. Nevertheless, the determining causes cannot at present 

 be precisely defined, and it is hardly probable that the plasmatic membrane 

 is simply the direct expression of the physical surface tension, which is 

 necessarily always present. The latter may be,, however, of decisive 

 importance in the formation of the membrane by means of the molecular 

 forces exerted by it, while, at the same time, contact with the medium 

 may cause the substances in the peripheral film to be precipitated in an 

 insoluble form, which redissolves when returned to the interior of the 

 plasma. Since a membrane is formed on every isolated fragment of 

 plasma, even when its vital activity is reduced to the lowest ebb, it seems 

 as if the actual exposure of the peripheral film induces the formation 

 of the plasmatic membrane, without involving anything of the nature of 

 a stimulatory process or stimulatory reaction. If this be the case, the 

 actual plasmatic membrane will in general be of such minimal size as 

 to be incapable of measurement, and theoretically, a single or double 

 molecular layer is sufficient for the maintenance of all the diosmotic 

 properties possessed by the plasmatic membrane. 



A hyaloplasmic border of measurable thickness can hardly be identical 

 with the plasmatic membrane throughout its whole extent, although the 

 mere existence of such a border, since it indicates that its internal surface 

 has the power of repelling granules, show r s that a certain similarity must 

 exist between its characters and those of a plasmatic membrane. The 

 peripheral hyaloplasmic layer, however, as has already been pointed out 

 (Sect. 7), must be of great service to the organism, owing to its power 

 of forming solid and permanent skeletal products. 



The medium with which the plasma is in contact will in general exert 

 some influence upon the formation of the plasmatic membrane, if the latter 

 originates in the manner described. It is not known, however, whether 

 the limiting membranes, formed when in contact with water, oil, saline 

 solutions, or air 1 , differ in their osmotic properties, and if so to what 

 extent. These and similar problems are of importance in connexion with 

 the maintenance of the identity of the nucleus, chromatophores 2 , and 

 other plasmatic organs, as well as with regard to plasmatic fragments 

 of a myxomycete, which have been ingested by the plasmodium of 

 another species and yet remain distinct. The latter case leads to the 

 important and still unsolved problem as to why it is that only plas- 

 modia of the same species fuse with one another 3 , a problem which 

 is also of the utmost importance in connexion with sexual and other 

 phenomena. 



1 On gas vacuoles, see Klebahn, Flora, 1895, p. 241 (Phycochromaceae) ; Engelmann, Zool. 

 Auzeiger, 1878, p. 152 ; Hermann, Handb. f. Physiol., 1878, Bd. i, p. 348 (Protozoa). 

 a Pfeffer, 1890, 1. c., p. 252. 

 3 Celakovsky, Flora, 1892; Erg.-bd., 1892, p. 212. 



