Lecture X. 77 



pyrenoid may usually be seen, and a nucleus lies in a central 

 mass of protoplasm filling the hollow of the chloroplast. The 

 tapered end of the cell is colourless like the beak of Chlamydo- 

 monas, and in this part may be seen, under favourable conditions, 

 two contractile vacuoles. Further, the similarity between these 

 cells and Chlamydomonas is strengthened by the possession of 

 two cilia which extend from the outer end of the cell into the 

 surrounding water. It is the vibrations of the cilia of all the cells 

 which confer motility on the sphere. An eye spot or stigma like 

 that of Chlamydomonas is found in those cells of the sphere 

 which lie round the anterior pole. On the whole we may regard 

 Volvox as an aggregation of cells similar to the individuals of 

 Chlamydomonas. The cell-walls of the cells have become gela- 

 tinous and fusing together form the hyaline wall of the sphere. 

 Using proper fixing (or killing) fluids and suitable stains it is seen 

 that a well-defined pellicle sharply delimits and covers the swollen 

 cell-walls at their outer surface and is reflected radially inwards 

 along their surfaces of mutual contact. The same treatment 

 establishes the fact that their internal walls gradually merge into 

 the central watery cavity. 



But although the limits of these gregarious cells are thus sharply 

 defined towards one another by the outer layer of their cell-walls, 

 careful observation of a tangential optical section shows that each 

 mass of protoplasm (protoplast) is connected with its adjacent 

 fellows by extremely fine fibrillae of protoplasm. These form a 

 network throughout the wall of the sphere, each protoplast or 

 cell occupying a nodal point of the meshes. Hence it is that 

 all the protoplasts of the sphere are directly connected with one 

 another and the protoplasm of the sphere is continuous through- 

 out. Generally these fibrillse are single ; sometimes, however, 

 several parallel fibrillse. connect adjacent cells. 



In our preliminary observations the steady progress of the 

 spheres through the water was noticed. A few trials, by turning 

 round the preparation and cutting off direct light, will convince 

 an observer that this motion is a response to the stimulus of light, 

 in this resembling the motion of Chlamydomonas. It is evident 

 that to direct the motion of Chlamydomonas the co-ordinated 

 motion of its two cilia is necessary and presumably the stimulus 

 received by the stigma is transmitted to the protoplasm at the 

 base of the cilia and there liberates energy to activate the cilia. 

 The motions of these cilia must be co-ordinated with the direction 

 of the light so that they exercise a turning force on the cell un- 

 less its axis points along the path of the illuminating beam. In 



