92 



PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 



An entirely different mode of division is found in some of the more 

 simple flagellates. Euglena, for example, divides without any rupture 

 of the nuclear membrane and without any definite mitotic figure 

 (see Fig. 10, p. 30). The chromatin is in the form of granules 

 distributed throughout the nucleus, and surrounding a central, deeply 

 staining, larger granule, the division centre. When the cell divides 

 this granule first divides into two equal parts, about which the 

 chromatin granules are equally massed, and it corresponds to the 

 entire mitotic spindle of metazoan cells. This type of nucleus (the 

 centronucleus) is quite common among the protozoa, and from it we 

 can trace the evolution of the mitotic figure of higher animal cells 

 through forms like noctiluca and the heliozoa. 



In some forms among the flagellates, and in some infusoria, there is 

 no definite nucleus, but the chromatin granules are distributed through- 

 out the cell unconfined by a nuclear membrane. This is the case with 

 some forms of tetramitus and with some ciliates like dileptus. In the 



Fig. 30 



Nucleus of Noctiluca miliaris in division. Tlie light streak through the middle is the 

 groove in which the central spindle lies. 



former, the chromatin granules collect about the division centre at the 

 time of cell division, and the nucleus then divides like one of the centro- 

 nucleus type. In the latter each of the separate granules divides, 

 although this does not mean that each granule is represented in 

 both daughter cells; on the contrary, only those granules pass into a 

 daughter cell that lie in the half of the parent organism represented 

 by that daughter cell. Division here is a means of keeping the 

 cjuantity of chromatin material and the active surface up to a 

 standard (Fig. 31). 



Budding differs widely from simple division, in its external appear- 

 ance, at least, for, in the majority of cases, the nucleus does not divide 

 until the daughter individual is nearly formed. In many rhizopods, 

 for example, the protoplasm swells out as a large protuberance from 

 the surface of the cell until it is quite as large as the parent cell, and 

 then the nucleus divides and the organisms move apart, each with a 

 nucleus and an equal portion of the protoplasm. This is the case in 



