43 p. KIDD. ' 



Double refraction was also observed in voluntary muscles 

 when contraction first appeared in them and in the case of 

 the thigh muscles small streaks of striated substance could 

 be distinguished at the same time on the cells. Similar facts 

 were observed in the case of fibres of the back and abdominal 

 muscles. 



In the muscle-fibres of the tongue, back, and thigh of 

 embryos of the white mouse, 10 — 12 mm. in length, the first 

 traces of double refraction were visible, but only in isolated 

 fibres, when already striation could be recognised. 



Similar phenomena Avere observed in the trunk and tail 

 muscles of tadpoles of Rana temporaria, though the presence 

 of highly refractive yolk-plates rendered it somewhat difficult. 



Engelmann regards it as an established fact that from the 

 time that striation is evident double refraction is also clearly 

 visible. Possibly in the case of the voluntary muscles of 

 vertebrates the double-refracting particles are present from 

 the commencement in the regular arrangement which they 

 manifest later. This would present an analogy to the stalk 

 muscle of Vorticella and to the vibratile cilia. In other 

 cases it seems, as was suggested for the heart muscle, that the 

 striation depends on a regular desposition of double refract- 

 ing molecules which were previously present but were dis- 

 tributed uniformly in the isotropous ground substance. The 

 uniform result of all Engelmann's observations on develop- 

 ment is to show that no appreciable interval of time exists 

 between the appearance of contractility and double refraction. 



The following general conclusion is drawn : 



" Contractility,wherever and in whatever form it may occur, 

 is associated with the presence of double-refracting, positive, 

 uniaxial particles, whose optic axis lies in the direction of 

 shortening." 



Attention is drawn to the agreement previously found by 

 Engelmann to exist between contractile and non-contractile 

 uniaxial positive tissue-elements, e.g. connective-tissue-fibres, 

 cell membranes, cuticulse, &c. 



This agreement is seen in the fact that both classes men- 

 tioned tend to shorten themselves by imbibition, in the direction 

 of their optic axes, or in other words swell up in a directioji 

 at right angles to their optic axes, much more distinctly than 

 in a direction parallel to the same axes. This peculiarity is 

 absent in tissue elements, otherwise very closely related to 

 them, which do not refract light doubly, e.g. elastic fibres and 

 membranes. 



