150 CYTOPLASMIC INCLUSIONS 



little nitrogenous materials — probably not more than one percent — to 

 account for any significant part of the total excretion of Paramecium. The 

 application of these results to all Protozoa is by no means certain, since 

 I have shown previously (1933) that the pellicle of the Ophryoscolecidae 

 is highly impermeable and the only pathway for the excretion of kata- 

 bolic wastes is through the contractile vacuoles. Frisch (1938) has 

 demonstrated a similar impermeability of the pellicle in Paramecium 

 itself, and suggests that Weatherby's experiments be repeated, with the 

 use of more delicate tests which have been devised recently. 



The materials concentrated in the osmiophilic structures of the con- 

 tractile vacuole are not salts, since no significant accumulation is demon- 

 strated in the nephridialplasm by microincineration (MacLennan and 

 Murer, 1934). These materials may possibly be incidental in some 

 species, but Amoeba dies if this elimination is prevented (Mast and 

 Doyle, 1935b), demonstrating that these wastes are toxic and that in 

 this respect they have the known properties of the nitrogenous wastes of 

 metabolism. Evidence that these materials are the result of metabolism 

 is found in Ichthyophthirius (MacLennan, 1936). The vacuoles are 

 osmiophilic only during the active feeding phase, when large amounts of 

 food are being used and converted into storage bodies; but during en- 

 cystment, when these activities cease, the vacuoles are not osmiophilic. 

 The food of this protozoan consists entirely of epithelial cells, which are 

 largely proteins, but not more than a third of the reserves of the ciliate 

 are proteins, the rest being carbohydrate and fat. During the feeding 

 stage a large amount of the ingested protein would be deaminized to 

 form other reserves, with the result that much larger amounts of nitroge- 

 nous wastes would be formed during the feeding stage than in the en- 

 cysted stage. 



Frisch (1938) suggests that the contractile vacuoles also function in 

 respiration. This function cannot be correlated with the variations in 

 the nephridialplasm in various Protozoa and thus, if this is a function of 

 the contractile vacuoles, it is probably independent of the granules which 

 are being considered here. 



Lipoid Reserves 



The lipoid materials considered in this section include all lipoids 

 which are visible as granules and which are laid down during active 



