Lecture X. 79 



irregular chloroplast of large dimensions which confers a green 

 colour on the cell. The subsequent history of each of these 

 cells is as follows : the contents of the cell divide within its cell- 

 wall into four protoplasmic masses which do not entirely fill the 

 original wall but are aggregated round one of its poles. Each of 

 these four masses or cells now divide and eight cells are produced 

 which flatten themselves against the inside of the original cell- 

 wall so as to form a saucer-shaped mass. Growth accompanied 

 by repeated divisions and readjustments among these cells form- 

 ing the saucer finally convert it into a hollow sphere with a pore 

 leading into it. This pore occupies the pole opposite the centre 

 of the original saucer-shaped group. During these developments 

 the spherical group, now recognisable as an embryo, projects into 

 the cavity of the parent Volvox, as do those other metamorphosed 

 cells in which the same processes have taken place. The cells in 

 the walls of these embryos multiply and their cell-walls swell and 

 become gelatinous. 



Finally the development of cilia from its outer aspect converts 

 the embryo into a miniature sphere of Volvox rotating within the 

 cavity of its parent. The embryo then escapes with its fellows by 

 the disruption and death of its parent. As it escapes the enlarged 

 cells which are to form the coming generation may be plainly 

 distinguished among the vegetative ciliated cells which form its 

 wall. In fact, before the embryos escape, we may often see stages 

 in the development of three generations through the transparent 

 walls of the parents. In many spheres a clear cap, devoid of cells 

 and their protoplasmic connections, can be seen occupying the 

 posterior pole. This appears to be the persisting remnant of the 

 opening left by the incurving margin of the saucer-shaped embryo. 

 During subsequent development it has become closed by the 

 gelatinous material of the adjacent cell-walls. The cells provided 

 with stigmata are the direct descendants of the central cells of the 

 embryo, while the reproductive cells are descended from some of 

 the peripheral cells. When the embryo assumes the spherical 

 form the anterior region may be described as the light sensitive 

 hemisphere and the posterior region round the pore as the repro- 

 ductive region. The cells giving rise to these embryos, inasmuch 

 as they develop into new individuals without fertilisation, may be 

 called spores. 



Sexual reproduction is usually found in Volvox in spheres 

 distinct from those which carry on the production of spores. The 

 sexual individuals produce two kinds of gametes or sexual cells ; 

 sometimes both are formed in the same and sometimes in different 



