80 



PROTOZOOLOGY 



narily situated in the anterior region and appears as a reddish or 

 brownish red spot or rod, embedded in the cortical layer of the 

 cytoplasm. The color of the stigma is due to the presence of drop- 

 lets of haematochrome in a cytoplasmic network. The stigma is 

 incapable of division and a new one is formed de novo at the time 

 of cell division. In many species, the stigma possesses no acces- 

 sory parts, but, according to Mast, the pigment mass in Chlamy- 



Flagella 



Stigma 

 Pyrenoids 



Chromatophores 



— Nucleus 



Shell 



Chromatophores Pyrenoids 



Fig. 31. a, Trachelomonas hispida, X530 (Doflein); b, c, living and 

 stained reproductive cells of Pleodorina illinoisensis, XlOOO (Merton) ; 

 d-f, terminal cells of Hydrur us foetid us, showing division of chromato- 

 phore and pyrenoid (Geitler); g-i, Chlamydomonas sp., showing the 

 division of pyrenoid (Geitler). 



domonas, Pandorina, Eudorina, Euglena, Trachelomonas, etc., 

 is in cup-form, the concavity being deeper in the colonial than in 

 solitary forms. There is a colorless mass in the concavity, which 

 appears to function as a lens. In certain dinoflagellates, there is an 

 ocellus (Fig. 101,c, d,(j,h) M'hich is composed of amyloid lens and 

 a dark pigment mass (melanosome) that is sometimes capable of 

 amoeboid change of form. The stigma is, in general, regarded as 

 an organella for the perception of light intensity. Mast considers 



