186 University of ('alifornia Fiihlicafions in Zoologi/ [Vol. 16 



inhabiting the blood and tissues, are peculiarly subject to reactions to 

 the toxins of their hosts and possibly to endotoxins of their own pro- 

 duction, and on this account often present pathological conditions of 

 the cytoplasm and more especially of the nucleus. 



Another difBeulty arises when the attempt is made to state in 

 exact terms the meaning of the word nucleus. In general the nucleus 

 of the Protozoa is a very definite structure which can be pointed out 

 with certainty in almost every case, having a definite degree of or- 

 ganization that readily distinguishes it from extra-nuclear chromidial 

 bodies. This is far from being the ease, however, with the so-called 

 second nucleus of the Binueleata of Hartmann. 



The presence of extra-nuclear chromidia or chromatin in a cell, 

 even when having a definite position and persisting from one cell- 

 division to the next, cannot be considered as indicative of a binucleate 

 condition unless other facts, such as mitosis during division, point to 

 the nuclear behavior of the chromidia. 



Since it is known that some cytoplasmic structures, as the undulat- 

 ing membrane and axostyle of Trichomonas, the axostyle of Hexamitus 

 and Giardia, divide during the process of cell-division, the division of 

 an extra-nuclear chromidial body loses much of its significance. In 

 how far that division can indicate nuclear function is a question that 

 can be answered only by the most careful experiments and study of 

 a wide range of material. In the present state of our knowledge of 

 the chemical and physiological reactions of the various cell constitu- 

 ents of the Protozoa, the wide application of conclusions which at the 

 best must be tentative, is to be avoided. 



In the course of a series of investigations carried on during the 

 past two years on the protozoan parasites of various amphibian, rep- 

 tilian, and mammalian hosts, the subject of extra-nuclear chromatic 

 structures and chromidia, and their function in the cell, has claimed 

 a considerable amount of my attention. Certain new facts have been 

 thus brought to light in my investigations which seemed to demand, 

 for their explanation, a complete revision of the present status of 

 extra-nuclear chromatin structures and chromidia, and also of the 

 binuclear theory, so closely connected therewith. 



This has become a much discussed question within recent years, and 

 a large amount of literature has accumulated on the subject. It is a 

 question, however, on which theories may easily outrun facts. The 

 influence of Weismann 's ideas on the germ plasm has been largely the 

 cause of the attempt to find in these simple organisms the anlage of the 



