Notes. 553 



but since these zoospores or spermatozoa attach themselves to the 

 alga, it may possibly be a case of parasitism. 



Very frequently the four bodies of chlorophyll formed by 

 division do not immediately evolve into the parent type, but 

 escape and rest in masses of jelly. 



Occasionally three zoogonidia are found in a cell instead of 

 four (plate XIV. fig. 4). 



On the Optical Properties of Contractile Organs. 



By Doris L. Mackinnon, B.Sc, and Fred Vles. 



Within the last half century quite a number of writers have treated 

 of the optical properties of contractile organs (muscles, cilia, flagella, 

 etc.), ami have shown that these elements are illuminated between 

 the crossed nicols of a polarising Microscope. This optical reaction 

 has been considered as due to birefringence, and a certain number 

 of physiologists, among whom special mention must be made of 

 Engelmann, have believed that they could base certain very 

 important theories concerning contractility on the general fact of 

 the birefringence of contractile organs. But it is far from having 

 been strictly demonstrated that the illumination of all contractile 

 organs between crossed nicols is identical with a phenomenon of 

 birefringence ; as early as 1862 Eouget expressed his belief — in a 

 completely hypothetical fashion, and without demonstration in 

 support — that, in the case of muscle fibres, phenomena of 

 " depolarisation " by diffraction might easily simulate apparent 

 birefringence. This opinion, however, appears : scarcely to have 

 been submitted afterwards to thorough examination. 



One of us recently undertook (1908) the experimental investi- 

 gation of depolarisation phenomena in contractile organs, and 

 attempted to demonstrate that, while the illumination of muscle 

 fibres between crossed nicols is certainly due to birefringence, that 

 of vibratile cilia is of quite a different nature, and arises from a 

 phenomenon of partial depolarisation of the light by reflection or 

 refraction. The method for distinguishing between the two optical 

 phenomena consisted mainly in observing whether the illumination 

 of the object disappeared (depolarisation) or not (birefringence), 

 when this object was immersed in a liquid having the same index 

 of refraction as itself. The physical theory of depolarisation 

 states, in fact, that the illumination of a depolarising body varies 

 with the refractive index of the surrounding medium ; this illu- 

 mination disappears completely when the exterior index is the 

 same as that of the object (at that moment there are no longer any 



