INTRODUCTION. 11 
fantastic as they were varied. “Contractile fibres” 
served for a time to explain the phenomena of motion. 
Gruber advanced the theory of “ pressure from behind.” 
There was, he held, a push forward of the more fluid 
contents by the posterior ectoplasm, after the extrac- 
tion of water had given the latter a tougher consis- 
tency. The posterior extremity was “drawn into 
threads as the Ameba advances, and the effect of a 
reversal of the direction of movement, he said, was seen 
ina flow of ectoplasm from the posterior region, the 
more tenacious protoplasm appearing then on the oppo- 
site side.* 
Dr. Wallich disputed Gruber’s theory, and reiterated 
a view which he had long previously expressed, namely, 
Fig. 4.—Anastomosing pseudopodia of Biomyza vagans. x about 150. 
that the rush of granules is not dependent upon any 
previous contractile effort exercised in the posterior 
region. The flow merely takes the place of the ectosarc 
which has been suddenly projected forward. Hence, he 
argued, the motion is dependent on the contractile power 
of “the external sarcode layer, and the endosarc only 
passively participates in it.” + 
Calkins sums up the controversy by remarking that 
of late years, especially since Bitschh’s masterly work 
on the structure of protoplasm, there has been a general 
tendency to abandon the older theory of contractility, 
and to explain the movements of amoeboid bodies 
through the physical laws of liquids, and in particular 
the laws of surface-tension. The origin of a pseudo- 
* «Zeitschr. fur Wiss. Zool.,’ xl (1884). 
+ ‘Ann. and Mag. Nat. Hist.’ (5) xvi (1885), p. 215. 
