DIVISION RATE IN CILIATE PROTOZOA. 313 



with accompanying abnormally small size, and that entirely 

 normal differentiation has set in among cells which have been de- 

 rived by the abnormally early (rapid) multiplication of their 

 ancestors. 



We would suggest, therefore, that there may be, at bottom, 

 not any great difference between what shows itself in Guder- 

 natsch's work as differentiation, and the result which shows 

 itself as cell division in an animal where differentiation, so far 

 as it exists, can assert itself only intra-cellularly, and thus in a 

 very obscure manner. 



It is certainly entirely unnecessary to dwell upon the obvious 

 fact that the more nearly adult a metazoan animal is, the more 

 difficult it becomes to even suggest parallelisms which may exist 

 between it and unicellular organisms; so that, to discuss the 

 numerous physiological effects which have been obtained from 

 feeding thyroid tissues to various vertebrata, or from grafting 

 and transplantation experiments, or to examine the studies of 

 conditions in higher types provoked by pathological thyroid 

 growth and disease, is quite beyond the scope, if not impossible 

 in connection with the subject, of this paper. 



Summary. 



The conclusion to which the foregoing experimental results 

 point is that thyroid ingredients, no matter from what class of 

 vertebrates the gland be taken, produce essentially the same 

 result when given to ciliate protozoa {Paramcecium and Sty- 

 lonichia) as a food or as a factor in the medium in which they 

 live, viz., increased division rate. 



The tissue which has hitherto been used in experimental work 

 along this line has, we believe, always been taken from mammals. 

 We think it safe to say that, no matter how far apart taxonomi- 

 cally, or how distantly related phylogenetically the "higher" 

 and "lower" members of the vertebrate phylum may be, certain 

 physiological qualities in the thyroid glands are constant and 

 similar in all. 



Sufficient difference exists between the potential of the thyroid 

 secretion of one vertebrate class and that of another, so that, if 

 studies of the normal value of this gland are being made, glands 



