MORPHOLOGY AND BIOLOGY. 17 



chromidia. The amount of chromatin present within 

 the endoplasm is of considerable value in the differen- 

 tiation of species. 



Besides the nucleus , the endoplasm may contain 

 one or more vacuoles which may be contractile or 

 non-contractile in character. The nature of the 

 vacuoles is of great value in distinguishing the para- 

 sitic amoebae found in man from the free-living forms, 

 as a contractile vacuole is not present in the parasitic 

 amoebae, but is always present in most of the common 

 free-living species. Thus in the genus Entamceba a 

 contractile vacuole has never been observed in speci- 

 mens obtained from the human intestine and ex- 

 amined in the living condition, while in Amoeba Umax, 

 and Amoeba proteus, common free-living species, a 

 contractile vacuole is always present. It is significant 

 that the amoebae which have been cultivated from the 

 human intestine show in the cultural forms a con- 

 tractile vacuole, so far as I have observed. This fact 

 renders it more than probable that these amoebae were 

 really free-living forms which had been swallowed 

 with food or drink and had passed through the intes- 

 tinal canal in an encysted condition. 



The number and size of the vacuoles varies in 

 different species. Thus in Entamceba coli there is 

 seldom more than one or two small vacuoles present, 

 while in Entamceba Mstolytica, a number of vacuoles 



