ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 595 



is the only gland of internal secretion that has this effect. Boiling the 

 thyroid produces no change in the reaction. lodothyrin and iodine 

 fail to produce the thyroid effect. After prolonged thyroid treatment 

 individuals revert to the normal division-rate when returned to the 

 control medium. 



The life-history curves of the treated lines show the same depression 

 periods at the same time intervals as the control lines, and the thyroid 

 produces the greatest acceleration of the division-rate when the control 

 line is dividing most rapidly. 



The animals ingest and digest particles of the thyroid. There is no 

 significant difference between the number of gastric vacuoles formed in 

 a given time between the thyroid-fed and the control individuals. But 

 the thyroid-fed forms show profound disturbances of the exci-etory 

 system, and the contractile vacuoles may increase from two to three. 

 The thyroid-fed forms show at all times a highly vacuolated protoplasm, 

 such as has been described for starved individuals. This is true even 

 during the periods of most rapid division. It has been shown that these 

 large vacuoles are to be considered non-contractile excretory vacuoles. 

 It is suggested that the results of thyroid-feeding in Paramecium are 

 due to the presence of a remarkably stable hormone in the thyroid, 

 which may be classified among " dissimilatory " hormones. 



Reactions to Colours.*— S. 0. Mast has studied the relations 

 between spectral colour and stimulation in the lower organisms, e.g. 

 Eughna, various Monads, Eudorlna, larva of Arenicola cristata, earth- 

 worms, larvjB of blowfly. In the fifteen species studied stimulation by 

 light was found to depend upon the wave-length, i.e. certain spectral 

 colours are much more efficient as stimulating agents than others. If 

 the light in the spectrum on either side of the maximum be made 

 sufficiently intense it becomes more effective than that at the maximum. 

 There is no evidence indicating the presence of colour-vision in any of 

 the forms studied. Bees are the lowest forms in which colour-vision has 

 been clearly established. 



The distribution in the spectrum of stimulating efficiency differs in 

 some species that are closely related (Gonium and Pandorina) and is 

 essentially the same in some that are not closely related {Euglena and 

 Lumbricus). The shorter wave-lengths are relatively more efficient for 

 green plants than they are for any animals, and there is nothing in the 

 nature of colour-vision in any of the plants. The contention therefore 

 that the reactions to colours in plants and animals is the same is not 

 well founded, although some of the chromatic reactions in animals may 

 be essentially the same as those in plants. 



The distribution in the spectrum of stimulating efficiency in any 

 given species is continuously the same, regardless of changes in physio- 

 logical states, environment, and character of response, e.g. it is the 

 same in individuals when they are negative as it is when they are 

 positive. As to the nature of the chemical processes associated with the 

 responses no definite conclusions can be drawn. 



* Journ. Exper. Zool.', xxii. (1917) pp. 471-528 (4 figs.). 



