bo 
H. E. JORDAN 
This hypothesis takes two distinct forms. In one of these 
forms some relatively less fluid constituent is supposed to im- 
bibe a relatively more fluid constituent of the intrasarcostylic 
sarcoplasm, causing In consequence a swelling and shortening 
of the sarcomeres. In the other form, the swelling and shorten- 
ing of the sarcomeres are supposed to result from an imbibition 
of intersareostylic fluid, much in the manner in which a hempen 
rope swells and shortens (‘contracts’) when suspended in water. 
In an intermediate form this hypothesis occurs in Englemann’s 
conception of muscle contraction. Englemann‘ explains con- 
traction as the result of the absorption of the isotropic material 
of the sarcoplasm by the anisotropic constituent, the latter being 
segregated in the so-called ‘dim’ dise. But the successful 
operation of such a contraction mechanism requires suspension 
in a fluid whose temperature can be raised suddenly. A parallel 
of this muscle-contraction mechanism is believed to be given in 
a string of catgut suspended in water, the temperature of which 
can be raised suddenly by the passage of an electric current. 
This compromise theory of muscle contraction accordingly 
makes an intrasarcostylic imbibition process, on the part of the 
isotropic and anisotropic constituents, dependent upon an 
imbibition of fluid by the sarcostyle as a whole. 
Schaefer?’’ discards this thermodynamic hypothesis of muscle 
contraction, and explains contraction as the result of the passage 
of the relatively more fluid ‘hyaline substance’ of the clear disc 
into ‘pores’ within the less fluid ‘sarcous substance’ of the dark 
disc, thus effecting a horizontal widening and a longitudinal 
shortening of the sarcomere. But, as will be shown below, 
reference to Schaefer’s illustration of a so-called ‘contracted’ 
sarcostyle of the wasp’s wing muscle, upon which he bases this 
explanation, makes it clear that this supposed intrasarcomeric 
imbibition of a fluid by a semisolid substance demands that the 
sarcostyle be bathed by a hypotonic solution (not an isotonic 
medium, as normally), and this fact removes Schaefer’s hypoth- 
esis to the same category as that of Englemann. 
The later chief exponents of a frank imbibition hypothesis of 
contraction are McDougall and Meigs. The latter claims to 
