AUTONOMOUS LOCOMOTORY MOVEMENTS 537 



category ciliary and muscular movements as well. From the well-known 

 characters of muscular motion we might feel inclined to refer all these proto- 

 plasmic movements to contractility of the outer layer. When such an Essump- 

 tion was found to be indefensible, Hofmeister (1867) attempted to show that 

 protoplasmic movement was due to a change in the attraction for water of the 

 smallest protoplasmic particles, while Engelmann (1879) held that it was due 

 to alterations in their form. All these attempts to explain the phenomena attri- 

 bute to the protoplasm an unexplained character, which is taken for granted, 

 although it must apply to living protoplasm only and not at all to lifeless bodies ; 

 moreover, they remove the problem into the realms of the invisible. More recent 

 explanations (compare Jensen, 1902), such as those of Berthold (1868), 

 BtJTSCHLi (1892), and Quincke (1888), must be dealt with more carefully, 

 because they attempt to refer protoplasmic movement to purely physical causes. 



In general these theories assume that protoplasm is a liquid and that its 

 normal shape is a sphere. Variations from the spherical form and the move- 

 ments themselves are thus accompanied by alterations in the surface tension 

 [compare Ewart, 1903]. As a matter of fact, the protoplasm, after receiving 

 a wound or other injury, may frequently be observed to round itself off into a 

 sphere, and hence it cannot be doubted that certain parts of it at least are liquid 

 in character. Surface tension is undoubtedly a very important principle, but 

 we must not expect to solve by its aid, once for all, every problem connected 

 with protoplasmic movement. Moreover the authors referred to are not in 

 accord as to the details of the explanation. 



In order that we may obtain a superficial acquaintance, at least, with such 

 physical theories of protoplasmic movement, we will look somewhat more 

 closely at amoeboid movement only, and leave on one side ciliary motion and 

 the streaming that takes place within the cell, since these present greater diffi- 

 culties. We need not discuss Quincke's (1888) views, since they assume certain 

 conditions that are certainly not realized in the organism. We will therefore 

 limit ourselves to a consideration of Berthold's (1886) and Butschli's (1892) 

 theories as to the movements exhibited by such a form as Pelomyxa. Berthold 

 compares this amoeba with a drop of liquid spreading itself over, orround, asoHd 

 body or a drop of another liquid with which it cannot mix. Looking more especi- 

 ally at the former case, and considering a drop of liquid lying on a plate of glass, 

 the space which it covers will depend in the first instance on the surface tensions 

 between the glass and the liquid, the glass and the air, and the liquid and the air, 

 and this will vary especially according to the chemical composition of the liquid 

 and with the temperature. A homogeneous liquid spreads over the substratum 

 equally in the form of a lens ; if the glass be not quite clean, or if the drop be 

 heterogeneous, or if it be of a different temperature in different places, the form 

 of the drop is irregular and more extended in one direction than another. In 

 Amoeba, owing to chemical differences between anterior and posterior ends, 

 polarity is induced ; the anterior end alone extends in a thin layer, adhering 

 to the substratum, while the posterior end detaches itself from the substratum 

 when adhesion is reduced, and, owing to surface tension, endeavours to round 

 itself off. The extension of the anterior end takes place, according to Ber- 

 thold, with a certain amount of energy, it is pushed out, not pulled out and 

 the material required for this extension can be provided only out of such parts 

 as lie behind. ' Thus there arises a suction action of the centrally directed 

 current. As far as the anterior margin is concerned, the movement is 

 fan-shaped, because the extension is more vigorous in the centre than it is at 

 the sides.' Another factor of moment to be taken account of is the pressure 

 from behind, associated with the efforts on the part of the posterior margin to 

 round itself off. 



BtJTSCHLi advances first of all certain physical objections to this theory. 

 He shows that Quincke's views as to extension, on which Berthold bases 



