180 THE PROTOZOA 



nucleate by formation of secondary nuclei from the chromidia, as 

 described above (Fig. 32, p. 67). The secondary nuclei are entirely 

 distinct in their origin from the primary nuclei, which degenerate 

 when the secondary nuclei are formed. A multinucleate Arcdla 

 may reproduce itself by binary fission after division of each secon- 

 dary nucleus by karyokinesis ; of each pair of secondary daughter- 

 nuclei, one goes to one daughter-^! rcella, the other to the other, so 

 that each daughter-.4rceWa has the same number of nuclei exactly 

 (Hertwig, 65). 



Gemmation takes place in multinucleate forms containing a 

 number of secondary nuclei. A portion of the body-protoplasm 

 becomes centred round each secondary nucleus, and thus a small 

 cell is formed, which becomes amoeboid, quits the parent-body, and 

 either grows directly into an adult Arctlla by the successive stages 

 described above, or before doing so performs an act of syngamy. 



Gemmation, as above described, takes place in three different 

 ways, as follows : 



1 . The buds are formed one at a time, and the parent-individual 

 persists and continues to reproduce itself (simple gemmation, 

 Fig. 80, H). 



2. The whole body of the Arcetta breaks up into numerous buds 

 which swarm out of the shell, leaving behind in it the two primary 

 nuclei, with a small quantity of residual protoplasm. The parent- 

 individual then dies off, apparently, but it is possible that it may in 

 some cases regenerate the body again. This process of multiple 

 gemmation differs only, from the simple gemmation described in the 

 previous paragraph in being, as it were, greatly intensified, taking 

 place with such rapidity as to use up almost the entire protoplasm 

 at once (Fig. 80. /). 



3. The protoplasm of the Arcella, with the chromidia, streams 

 out of the shell, leaving in it only the degenerating primary nuclei. 

 Outside the shell the amoeboid body forms secondary nuclei, and 

 breaks up by multiple fission into a number of amoebulse. This 

 process differs from that described in the foregoing paragraph only hi 

 taking place outside the shell (Fig. 80, J). 



As already stated, the amcebulae formed by multiple gemmation 

 may either be agametes, which develop directly into the adult form, 

 or gametes, which first go through a process of syngamy whioh has 

 been described in the previous chapter (Fig. 80, K, L). Both 

 agametes and gametes arise in the same manner ; the gametes, 

 however, show sexual differentiation as regards size. The zygote 

 is an amoebula which develops into the adult form in the same way 

 as an agamete. In addition to syngamy (karyogamy) between 

 amoebula?, chromidiogamy between adult Arcettce also occurs, as 

 already described ; the result in this case also is the formation of a 



