ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 377 



mecium aurelia during four hundred and twenty generations. These 

 experiments have shown that the effect of the thyroid is to increase 

 greatly the rate of division, except at the time when the line was nearing 

 the close of its cycle. The effect disappears after feeding with thyroid 

 is stopped. Similar treatment with thymus gave negative results. It 

 was shown that the Infusorians actually ingest and probably digest 

 particles of thyroid. It is suggested that the effect of the thyroid is 

 primarily upon the metabolic activities of the cell, and, further, that at 

 the close of the life-cycle some element of the protoplasm necessary to 

 digestion is lacking, thus inhibiting the effect of the thyroid. 



Feeding Reactions of Bursaria.* — E. J. Lund has studied selection 

 in ingestion and extrusion of particles in this Infusorian. It has three 

 ways of rejecting solid particles. These are (a) the path of total 

 rejection, shown by particles that never enter the oral apparatus ; 

 \b) the path of rejection of large particles, which are made to retrace 

 the path by which they entered ; and (c) the path of rejection of small 

 particles, which leave the oral pouch by way of the base of the oral 

 sinus and are passed backward over the ventral side of the body. 



No definite path is followed by the food-vacuoles during digestion, 

 and in their passage through the cytoplasm. Residues are got rid of 

 from a small area on the mid-dorsal side of the cell. The amount of 

 food eaten and the rate at which it is eaten depends on the physiological 

 state of the cell, i.e., " the condition as a whole, of the equilibria in the 

 physical and chemical reaction system, the cell, at a certain time in the 

 duration of its existence." The rate of feeding is not affected in pro- 

 portion to the concentration of the food (yolk-grains) in suspension. 

 Mechanical stimulation decreases the rate of feeding or inhibits it, 

 roughly in proportion to the degree of stimulation. Rise in tempera- 

 ture increases the rate of feeding on yolk. Continuous action of white 

 light of high intensity had no detectable effect upon feeding on yolk. 

 Feeding may continue during stimulation by a direct electric current 

 of sufficient intensity to control the direction of movement of the 

 organism. 



The Infusorian can discriminate between and select non-toxic grains 

 of yolk from among toxic ones. " Whether or not it will eat grains 

 that have absorbed a soluble substance depends upon (a) the steepness 

 of the effective concentration gradient of the dye, between the grain 

 and the non-toxic medium ; and this in turn depends upon the amount 

 of dye absorbed which is subject to a reversible a 1 (sorption; (b) the 

 specific chemical properties (taste ?) of the substance absorbed." 

 Different parts of the cell seem to be affected unequally by certain toxic- 

 substances. Vacuoles containing indigestible substances are soon ex- 

 truded, while those containing food are retained ; so there appears to be 

 selective extrusion as well as selective ingestion. 



Digestion in Bursaria.f — E. J- Lund has studied the processes of 

 digestion and absorption in the food vacuole of this Infusorian. and has 



* Journ. Exper. Zool.. xvi. (1914) pp. 1-52 (8 figs.). 



t Journ. Exper. Zool., xvii. (1914) pp. 1-42 (2 pis. and 8 figs.). 



