452 



BIOLOGY OF THE PROTOZOA 



division of the nucleus, without complete cell division, becomes 

 binucleated or multinucleated. In a similar matter other cham- 

 bers are added to those already formed until complicated aggre- 

 gates measuring 3 or more inches in diameter in some cases result 

 (Nummulites, etc.). These, however, are to be regarded as single 

 individuals of syncytial nature illustrating growth and differentia- 

 tion rather than reproduction. With the formation of a brood of 

 reproductive bodies each of which produces a similar multinucleated 

 individual we can speak of asexual reproduction in a strict sense. 

 Thus in Polystomellina crispa (Fig. 123, p. 235), after multiplication 

 of the nuclei, the latter give rise by fragmentation to a large number 

 of minute nuclei having the significance of chromidia. The plasm 

 forms islands about each of these minute nuclei, or groups of them, 

 and is then broken up into as many minute cells as there are islands. 



B 



C 



Fig. 187. — Types of polythalamous Foraminifera shells. A, nodosarine type; B, 

 frondicularian type; C, spiral type. (After Carpenter.) 



These small cells, in the form of amebulae or amebospores leave 

 the parent shell by way of the foramina or by the mouth opening of 

 the last chamber and after a short period of ameboid movement 

 settle down and secrete the characteristic shell chamber. This 

 initial test (proloculum) is measurably larger than the initial cham- 

 ber of the organism which formed the amebulae and is called a 

 macrospheric chamber as opposed to the microspheric chamber of 

 the first generation. A new multi-chambered shell is then formed 

 according to the type of structure of the species. When fully grown 

 the protoplasm of this macrospheric generation breaks up into a 

 swarm of small biflagellate flagellispores which leave the parent 

 shell and swim about by means of their flagella. These flagellates 

 are gametes which ultimately unite two by two to form zygotes. 

 The flagella are absorbed and the young zygote secretes the shell 

 material of the first chamber about which other chambers are 



