418 Prof. Th. W. Engelmaim. [Mar. 14, 



Liglit lux optimum reagens, as Buys Ballot said solves this ques- 

 tion for us. If we examine tlie optic properties of contractile fibrils,. 

 with the aid of the polarising microscope, we find that all of them 

 are double -refractive, with one optical axis parallel to the direction 

 of contraction. This property, long ago discovered by Boeck in the 

 striated muscles of the higher animals, has now been proved to apper- 

 tain to the muscular fibres of all the animals examined for that end,, 

 to the contractile fibrils of Protozoa, e.g., to the muscle of the stalk 

 and the myonemata in the subcuticular layer of Vorticella, to all 

 kinds of ciliary apparatus, and in many cases to contractile proto- 

 plasm also. 



This general occurrence of double-refracting power is the more 

 indicative of relations to contractility, since non- contractile cells, as. 

 a rule, lack double refraction, even where we meet with a fibrillar 

 structure, as in the axis-cylinder of a nerve-fibre. 



Our conjecture gains, I believe, a very high degree of probability 

 by the following series of observations, which are the more convincing 

 since they relate to quite different categories of phenomena, inde- 

 pendent of each other. 



In the first place, there is the fact that contractility and double 

 refraction in the course of ontogenesis always appear at the same time,, 

 e.g., in the heart of the chick on the second day of incubation ; in the 

 muscles of the trunk and skin on the fifth or sixth day; in the 

 muscles of the tails of tadpoles when the length of their body is 

 3 to 4 mm. ; in the muscles of the stalk of Vorticella, and in cilia 

 so soon as these organs become visible. 



Other evidence seems to me to be afforded by the behaviour of 

 the striated muscles. Here the fibrils consist of the doubly-refractive 

 sarcous elements and the singly-refractive material which joins these, 

 the two alternating regularly. The two are wholly different a& 

 regards their optical, mechanical, and chemical properties ; and these- 

 properties, moreover, during contraction, change in an opposite- 

 way. Hence the functions of the two must be different. And 

 since the changes of form, volume, &c., of the doubly-refractive- 

 parts during contraction prove that in each case these parts must 

 be the seat of contractile power, the singly-refractive junctions will 

 most probably have another function. "We will come back to these 

 changes further on. 



A third evidence is afforded by the observation that the specific 

 force of contraction in different muscles is, in general, greater, the 

 better developed the power of double refraction, comparison, of course, 

 in each case being made with parts of the same thickness. 



In the development of the pseudo-electric organs of Raja out of 

 striated muscular fibres, one of the signs of the incipient change of 

 structure and function is the vanishing of double refraction in the 



