I04 THE AMOKBAE LIVING IN MAN 



respects, and it is possible that the organism may eventually be found to 

 belong to the same genus. 



Description. 



Endolhnnx nana has already been described more or less completely 

 by Wenyon and O'Connor (1917), Swellengrebel and Mangkoe Winoto 

 (1917), and Kuenen and Swellengrebel (1917). I have also given a 

 brief account of it in an earlier joint paper (Dobell and Jepps, 1917), and 

 shall therefore now merely note the points of chief importance. 



E. nana is a small amoeba which usually measures, when rounded, 

 from 6 /ti to 12 /i in diameter. Living amoebae average about 8 jjl, but 

 fi.xed and stained specimens are generally about i n less in diameter. 

 The living forms somewhat resemble small free-living amoebae (so-called 

 " Umax " amoebae) at first sight, but this resemblance vanishes on closer 

 study. They possess no contractile vacuoles, their nuclei are not clearly 

 visible when alive, and their movements rapidly cease outside the body. 

 Such movements as are usually observable under the microscope are like 

 those of a small individual of E. colt — sluggish creeping, with few blunt 

 pseudopodia showing a variable degree of distinctness between endoplasm 

 and ectoplasm, soon followed by mere change of shape without locomo- 

 tion, and ending in cessation of all movements. The cytoplasm usually 

 contains many food-vacuoles filled with micro-organisms. 



The most characteristic feature of the free amoeba is its nucleus, 

 which can only be studied satisfactorily in well fixed and stained 

 specimens from very freshly passed stools, and under high powers of 

 the microscope. The nucleus is vesicular, and measures from about 

 I /A to 3 ^ in diameter, according to the size of the individual. As a 

 rule its diameter lies between 2 fi and 2*5 fj,. (See PI. I, fig. 7, and 

 PI. II, figs. 18-23.) There is a peculiar karyosome in the nucleus of this 

 organism, distinguished by the great diversity of form which it displays. 

 It contains most of the nuclear chromatin and consists usually of one 

 fairly distinct mass connected by threads or processes with one or more 

 smaller masses. The main mass of the karyosome is usually eccentric 

 in position, and consequently the organism often appears, when deeply 

 stained or when seen under a comparatively low magnification, to con- 

 tain a nucleus with a single rather small and eccentric karyosome. 

 Figures of some of the commoner forms of nucleus are shown in 

 PI. I, fig. 7, and PI. II, figs. 18-23 ; a"d fig. 24 (PI. II) shows 8 other 

 nuclei from other individuals. These drawings will convey a clearer 

 idea of the nuclear peculiarities of this species than pages of description. 

 It should be added that very careful study of the nuclei of these 

 organisms — in really good preparations, and with the best lenses and 

 critical illumination — shows that hardly any two individuals present 

 precisely the same nuclear appearance. Several distinct types of nucleus 

 can be distinguished — such as those represented by figs. 18, 19, 22, etc. — 

 but there are many variants and intermediates, and the appearances 

 differ, of course, according to the orientation of the nuclei in relation to 

 the observer. 



Apart from its karyosome the nucleus of E. nana presents no remark- 

 able features. It possesses a well-marked nuclear membrane in which 

 minute granules — possibly of chromatin — can sometimes be seen (figs. 

 18 21, etc.) ; and between the membrane and the karyosome there is 



