Lecture V. 37 



the central protoplasm. As we focus it and examine it, it becomes 

 evident that it is a minute bag formed of a transparent membrane 

 within which are rather distinct granules. It is in fact the nucleus 

 of the cell and is embedded in semi-fluid granular protoplasm 

 occupying the central region. The colour of this central region 

 is evidently conferred on it by the light transmitted through the 

 peripheral regions : just as colourless water in a green wine-glass 

 appears green. The colourless" central protoplasm is continuous 

 with the colourless protoplasm of the beak and the cilia. It will 

 also be seen by careful focussing that a very thin colourless layer 

 of protoplasm covers over, like a skin, the coloured zone. In fact 

 the coloured region of the protoplasm of the cell has the form of 

 a bell open towards the beak. This special coloured bell-shaped 

 mass of protoplasm is called a chromatophore, and, having formed 

 the green pigment chlorophyll, this chromatophore is styled a 

 chloroplast. The layer of protoplasm forming the surface of the 

 protoplasm of the cell and in immediate contact with the cell- 

 wall is free from granules, while the central protoplasm and that 

 at the base of the beak is finely granular. At some point in the 

 surface layer a very minute lens-shaped body may be seen. It 

 has a very high refractive index and hence appears almost black 

 if the diaphragm is closed, but it reveals a fine crimson colour 

 when the diaphragm is opened and a wide cone of light illuminates 

 the cell. This crimson body is called the stigma or eye-spot. It 

 is usually located in the outer protoplasm near the beak or 

 anterior end of the cell, and it appears circular or ovoid according 

 as it is seen in plan or in profile. 



While making these observations on the structure of the cell of 

 Chlamydomonas, it is probable that the attention of the observer 

 will be attracted by the periodic appearance in the protoplasm of 

 the beak of one or two spaces or drops of fluid. These are at 

 first almost invisibly small, they then grow and suddenly disappear. 

 It is as if they slowly swelled until they passed some limit and 

 then exploded. These are the contractile vacuoles. There are 

 usually two, one at each side of the beak, but often they are not 

 visible simultaneously. 



The closed end of the chloroplast is turned towards the posterior 

 pole of the cell. At this point the chloroplast is much thicker 

 than at the open end, and occupying this thickened mass at the 

 closed end is a structure called the pyrenoid. It i's an accumu- 

 lation of protein and starch. The starch is in the form of granules 

 surrounding a central mass of protein. 



If these observations have been carried out facing a good 



