GENERAL DIFFERENTIATION OF THE PLANT-BODY 



a 



number of its cells is much greater, and so is the size of the whole colony. In 

 Volvox aureus (Fig. 4) the number of cells varies between 200 and 3,000, in V. globator 

 the number in the asexual colonies, which are the only ones now under consideration, 

 varies between 1,500 and 16,400. The remarkable inner structure of the colonies 

 in this genus cannot here be described, suffice it to mention that they possess 

 two poles which are distinguished as well by the history of their development as 

 by their ultimate structure. The one of these, the trophic pole, which is in front 

 when the colony is in motion, marks the point at which the union of the cells 



took place by which the 

 sphere was produced out 

 of a concave surface, and 

 sometimes an opening may 

 be found at this point. The 

 cells in the vicinity of the 

 trophic pole are nourish- 

 ing cells, whilst the pos- 

 terior half of the colony 

 consists of cells by which 

 the function of propagation 

 is performed, this being 

 limited in each colony to 

 either the sexual or the 

 asexual method. The cells 

 at the anterior pole have 

 a very large red eye-spot, 

 the stigma, which evidently 

 plays a part in the disposi- 

 tion of the movements of 

 the colony, and these eye- 

 spots diminish in size in 

 the cells towards its equator, 

 and disappear altogether 

 in those on the posterior 



FIG. 4. Volvox aureus. A old colony showing daughter-colonies I, and . _. , 



antheridia a, and eggs o. The cells appear as small circles upon the_ halt Or ai'C 1'epiaCCQ Dy 



surface of the sphere. Magn. 180. B young colony. Magn. 687. C , , ., j y n 



young- colony seen from the side. D isolated spermatozoids. Magn. 824. a COlOUnCSS l-urop. 



the posterior half of the 



colony all the cells are not propagative cells in Volvox globator there are usually 

 only eight of these ; the function of the other cells is purely nutritive, and at a 

 later period they perish, being probably used up by the propagative cells, and 

 in correspondence with this their protoplasmic body stops growing during the 

 development of the propagative cells, and becomes poor in substance. This 

 far-reaching influencing of one cell by another is explained by the fact that 

 there are protoplasmic continuations between the cells, and these are more 

 numerous between the cells in the posterior generative hemisphere than in the 

 anterior trophic one. In Volvox then a division of labour has taken place; first 



