ENGELMANN ON CONTRACTILITY AND DOUBLE REFRACTION. 39 



stalk muscles behaved themselves like positive, uniaxial, 

 double-refracting elements, the optic axis being parallel to the 

 long axis of the fibres. 



Tracing the development of the Zoothamnium, Engel- 

 mann found that the formation of the stalk muscles com- 

 mences regularly when the stalk is about '4 to *6 mm. long. 



The muscle appears in the form of a flat highly refractive 

 band, about 014: mm. in breadth, in the axis of the stalk. 



From the very earliest point of its appearance it possessed 

 contractility as well as double refraction. At the same time 

 the protoplasmic mass surrounding the muscle appeared in 

 the form of a stripe of finely granular substance about "008 

 nun. in breadth, lying directly on one surface of the muscle. 

 Engelmann considers this protoplasmic granular matter to 

 be formative material for the development of the wall of the 

 stalk as well as of the stalk muscle. The negative results 

 of those who had previously studied the stalk muscle seem 

 to have depended on the exceeding thinness of the elements. 



In specimens of Carchesium j^olypimini, Vorticella nehu- 

 lifera, and Zoothamnium aselK it was impossible to mis- 

 take the anisotropous condition of the stalk muscle, or, 

 indeed, the much le&s definite though similar appearances 

 presented by the wall of the stalk, especially in a coloured 

 field. 



In the case of Stentoi' polymorphus and Stentor Roeselii 

 the cortical layer was proved to be double refracting, but 

 owing to the impossibility of isolating the muscle-fibrils, in 

 the natural condition, from the protoplasm of the same layer 

 with which they are in contact, direct proof is wanting of the 

 double-refracting capacity of these fibrils. 



In different forms of Epistilis clear evidence was obtained 

 of double refraction in the fibril layer, Avhich in places is so 

 frequently so far removed from the cuticula that any mistake 

 in consequence of the double refraction of this latter can be 

 excluded. 



3. Cilia and Spermatozoa. — Under favorable conditions 

 Valentin found that cilia are double refracting. Valentin's 

 statements relate to the ciliated cells of the respiratory 

 mucous membrane of the marmot, of the oral cavity of the 

 frog, of the gills of the common mussel, and in Opalina 

 ranarum. 



Engelmann prefers the ciliated cells of the larger Rotifera 

 and the large cilia on the gills of bivalves. 



But he found that all the different cilia behave like uniaxial, 

 positive double-refracting elements, whose optic axis is parallel 

 to the long axis of the cilia. 



