56 PROTOZOA 



Reproduction l:)y tission has been clearly made out in most 

 members of the group ; some of the multinucleate species often 

 abstrict a portion, sometimes at several points simultaneously, 

 so that fission here passes into budding^ (Fig. 9, 2, 6). 



Brood-division, either by resolution in the nmltinucleate 

 species, or preceded by multiple nuclear division in the habitually 

 1-nucleate, though presumably a necessary incident in the life- 

 history of every species, has only been seen, or at least thoroughly 

 worked out, in a few cases, where it is usually preceded by 

 encystment, and mostly by the extrusion into the cyst of any 

 undigested matter.- 



In TTichosphaerium (Fig. 9) the cycle described by Schaudinn 

 is very complex, and may be divided into two phases, which we 

 may term the A and the B subcycles. The members of the A 

 cycle are distinguished by the gelatinous investment being armed 

 with radial spicules, which are absent from the B form. The 

 close of the A cycle is marked by the large multinucleate body 

 resolving itself into amoeboid zoospores (3), which escape from 

 the gelatinous test, and develop into the large multinucleate 

 adults of the B form. These, like the A form, may reproduce by 

 fission or budding. At the term of growth, however, they 

 retract their pseudopodia, expel the excreta, and multiply their 

 nuclei by mitosis (7). Then the body is resolved into minute 

 2-tlagellate microzoospores (8), which are exogmnous gametes, 

 i.e. they will only pair with similar zoospores from another 

 cyst. The zygote (9-11) resulting from this conjugation is 

 a minute amoeboid ; its nucleus divides repeatedly, a gelatinous 

 test is formed within which the spicules appear, and so 

 the A form is reconstituted. In many of the test-bearing 

 forms, whether Lobose or Filose, plastogamic unions occur, and 

 the two nuclei may remain distinct, leading to plurinucleate 

 monsters in their offspring by fission, or they may fuse and form 

 a giant nucleus, a process which has here no relation to normal 

 syngamy, as it is not associated with any marked change in the 

 alternation of feeding and fission, etc. In Tricliosphaerium also 

 plastogamic unions between small individuals have for their only 

 result the increase of size, enabling the produce to deal with 



1 See Lauterborn in Z. luiss. Zool. lix. 1895, pp. 167, 537. 



- C. Scheel has seen Amoeba j)rotcus produce a brood of 500-600 young 

 anioebulae, wliich he reared to full size (in Fcstsclir. f. Knpffcr, 1899). 



