408 ZOOLOGY. 



has been supposed to be an organ of vision. Taken 

 literally, of course, this is an absurd supposition, but, on 

 the other hand, it is possible that the pigment, by absorb- 

 ing light, may cause the protoplasm to be specially stimu- 

 lated by luminous rays at this point, and this function if 

 it exists may be regarded as similar to the primary function 

 of an organ of vision in higher animals. The stigma con- 

 sists of a special portion of the cytoplasm containing a 

 large number of minute red granules of a substance called 

 haematochrome. This substance is, like chlorophyll, soluble 

 in alcohol and ether. It is insoluble in ammonia and acetic 

 acid. In the species called Euglena sanguinea haemato- 

 chrome is often present not merely in the stigma, but in 

 the general cytoplasm, in such quantity that the whole cell 

 has a red instead of a green colour. The cell in this 

 species appears in fact to pass regularly from a green to a 

 red condition ; it is always full of haematochrome when 

 dividing, and it has been suggested that the haemato- 

 chrome is produced by modification of the chlorophyll. 



5. Nucleus. The rest of the interior of the body 

 of the cell consists of protoplasm containing a nucleus and 

 certain other structures, of which the principal are the 

 chlorophyll bodies and granules of a substance called 

 paramylum. In Euglena viridis the nucleus is situated 

 near the pointed posterior end of the cell, and in the living 

 condition appears merely as a large colourless space in 

 which no details of structure can be distinguished. (See 

 Fig. 1, A, B, c.) It can be studied further in specimens 

 which have been killed and fixed in absolute alcohol and 

 stained with acid fuchsin. The nucleus is then seen as a 

 spherical vesicular body containing a dense central 

 nucleolus, which has been called a nucleo-centrosome, 

 and around it about 40 chromatic (i.e. stained) granules 

 scattered in the nucleoplasm (Fig. 203, D). 



6. Chloroplasts, Byrenoids, and Paramylum. It is 

 characteristic of Euglena viridis that the chlorophyll 

 corpuscles are of an elongated rod- like form and that they 

 radiate in all directions from a central mass of granules, 

 which, according to Dangeard, constitute a large pyrenoid 



