PKOTOCOCCUS PLTJVIALIS. 533 



which a central, also cellular, mass of protoplasm, or a 

 primordial-sac-like organism, performs the part of a 

 nucleus. 



The most distinctive characteristic of the primordial 

 cell, and what appears to constitute its most essential 

 importance in the life of the cell in general, but particu- 

 larly in that of the Zoospore (schwarm-zelle), consists in 

 its being the contractile element of the vegetable or- 

 ganism, that is to say, that from an intrinsic activity it 

 possesses the faculty of altering its figure, without any 

 corresponding change in volume. 



It was Ehrenberg who first asserted that there was an 

 absolute boundary between animals and plants ; finding 

 even, as he fancied he did, in the smallest of the former, 

 the Infusoria, which had previously been regarded 

 as mere unorganised masses of mucus, the same systems 

 of organs as those by which the most highly-developed 

 animal is characterised, that is to say, distinct nutritive, 

 motile, vascular, sexual, and sensitive systems. Siebold 

 called the existence of these organs in question, regarding 

 the organisation of the Infusoria as a homogeneous pa- 

 renchyma, in which he recognised only a nucleus, and in 

 one division a mouth and oesophagus. Nevertheless he 

 asserted that plants and animals were essentially distinct, 

 and that there was no transition from one to the other, 

 the nature of the plant being always immotile and rigid, 

 whilst the animal possessed the faculty of contracting and 

 expanding its body. This contractility, he observes in 

 another place, is the only certain diagnostic character, all 

 others being invalid. 



It is not, however, the animal organism itself which is 

 contractile, but only a single tissue in it ; all the rest, skin, 

 bones, and connective tissue, are as rigid or passive as 

 the vegetable membrane, or at most elastic ; in the higher 

 animals the muscles only are contractile, and only in the 

 lowest, viz. the Infusoria, the entire body. 



Whence Ecker assumed the existence of a special con- 

 tractile substance, which sometimes occurs in a formed 



