528 University of CaUfornkt. Publications in Zoology [Vol. 20 



in the formation of cysts and that no regular periodical life cycle is 

 apparent for parasitic amoebae. Kofoid (1923) has suggested that 

 encystment occurs as the result of the presence of reserve food rather 

 than for the purpose of protection to the protozoan. Protection is 

 afforded while passing from one animal to another, but this is a second- 

 ary function following from the formation of the cyst. As cysts are 

 found in the caecum as well as in the faeces, there are indications that 

 reproduction by multiple fission within the cyst in Council mania, may 

 occur within the host independent of discharge. The fact that budding 

 from the cyst apparently in the caecum of the host in which the cysts 

 were formed has been found in Councilmania muris is further evidence 

 in support of this theory. 



Mitosis and the mechanism of heredity. — Early workers in proto- 

 zoology expressed the opinion that cell division among the Protozoa 

 was affected by amitosis. Perfection of cytological methods and 

 further detailed work have resulted in the description of cell division 

 by the process of mitosis for one after another of the Protozoa that 

 had been previously thought to divide amitotically. It is verj^ prob- 

 able that mitotic cell division is the method of cell division in all 

 normal cells in the animal kingdom and that amitotic division must 

 be reserved merely to explan pathogenic or abnormal cell division. 



If the mechanism of heredity is bound up in the complicated 

 structures recognizable in mitotic division, one would expect to find 

 a hereditary mechanism in the Protozoa similar to the one known 

 in the Metazoa. Among the amoebae parasitic in man, accounts of 

 mitosis have been given for Endamoeba dysenteriae by Dobell (1919), 

 though he did not record the complete mitotic cycle, by Kofoid and 

 §wezy (1921) for Councilmania lafletiri, and by Swezy (1922) for 

 Endamoeba coli. 



These accounts show that a characteristic spindle is formed in the 

 amoebae with a centrosome at either pole, and that the centrosomes 

 are connected by an intradesmose (Kofoid and Swezy, 1921). The 

 centrosomes are formed from chromatin material within the karyo- 

 some. The nuclear membrane remains intact throughout the whole 

 process of division and merely lengthens and constricts in the middle, 

 finally dividing into two parts at the close of the telophase. 



In the present investigation the process of mitosis has been recorded 

 for Councilmania muris and C. decumani. It is essentially the same 

 in these two species and similar to that in Endamoeba coli and C. 

 lafletiri, the chief difference being that the chromosomes are smaller 



