CHROMIDIA AND NUCLEI OF A PROTOZOON 215 



nucleolus and a nuclear membrane. A circle of gas vacuoles 

 is often seen round the pylome, or they may be united into 

 one eccentric vacuole. One or more contractile vacuoles 

 are also usually present. The animal does not completely 

 fill its shell with which the ectoplasm is in contact only at 

 a number of points. 



In the ordinary binary division the two nuclei divide 

 simultaneously by a simple form of mitosis, 3, after part of 

 the animal's body has been protruded through the pylome. 

 The two new nuclei with other cell-elements pass into the 

 protruded part which secretes a new shell, and soon separates 

 from the parent organism. 



When an Arcella is viewed from above the outline of the 

 pylome shows as an inner doubly-contoured ring, and 

 another and very important feature is seen the chromidium, 

 4 and 5, Chr. In the vegetative state of the animal the 

 chromidium consists of a circle of rather coarse granules, 

 which pass to the outer side of the nuclei with a little space 

 between. In winter the chromidium plays its part in 

 reproduction in various ways. First, the fine granules of 

 chromatin, hardly noticed between the other granules of 

 the resting chromidium, become larger and run together to 

 form new nuclei. The subsequent fate of these is indicated 

 in Fig. 61, 5 to 11. 



In certain conditions Arcella encysts, forming a dense 

 cyst-wall which blocks the pylome. Before encystment 

 occurs food-remains (diatom shells) are extruded into the 



