28 



THE INVERTEBRATA 



usually closely precedes syngamy, as in the Metazoa, but in the 

 Telosporidia and Volvocina it takes place in the first division of the 

 zygote, so that for the whole of the rest of its life history the organism 

 is haploid. 



In many protozoa there are present in the cytoplasm, scattered or 

 massed into a group, numerous granules which, like chromatin, take 

 basic stains and are known collectively as the chromidium (Fig. 22 A). 

 They appear to arise from the nucleus, and have been said, but 

 probably incorrectly, in some cases to give rise to nuclei by con- 

 densation. They may appear upon occasion or be present through the 

 greater part of the life cycle. Their function is uncertain and probably 

 not always the same. 



Th.Q. fission of the Protozoa takes place in several ways. Whether in 



Fig. 22. Arcella. After Swarczewsky. A, Vegetative individual. B, In- 

 dividual in process of budding, chm. chromidium ; nu. nucleus ; op, opening 

 of shell ; sh. shell. 



asexual reproduction or in the formation of gametes, it may be: 

 {a) equal binary fission, the familiar mode of division of Amoeba, 

 Paramecium, and a vast number of other cases; {b) budding, in which 

 one or more small products separate from a parent body, as in 

 Arcella (Fig. 22 B), the Suctoria (Fig. 96), etc.; {c) repeated fission, 

 in which equal divisions give rise to four or more young which do not 

 separate till the process is completed, as in Chlamydomonas (Fig. 23), 

 the microgamete formation of Volvox (Fig. 44), etc.; or (d) midtiple 

 fission, in which the nucleus divides several times without division 

 of the cytoplasm, which finally falls into as many parts as there are 

 nuclei, usually leaving behind some residual protoplasm, which may 

 contain nuclear matter. Multiple fission is seen in the spore formation 

 of numerous protozoa, as Amoeba, sporozoa (Fig. 74 C, D; Gg, G3; 

 K, L), etc. The fission of multinucleate protozoa, such as Actino- 

 sphaerium, Opalina (Fig. 86), etc., to form multinucleate offspring by 



