ASEXUAL REPRODUCTION 



321 



differentiation among the cells. A few merely cease to 

 grow, but most of them develop a number of small motile 

 reproductive cells. This is brought about by a series of 

 divisions of the nucleus until there are about 8 (or some- 

 times 16) in each cell. The cytoplasm then divides up 

 into an equal number of portions, so that each nucleus is 

 surrounded by a small amount of individualized cyto- 

 plasm. Next each of these units undergoes a process of 



Fig. 80. — Ulothrix. A, part of a vegetative filament one of whose 

 cells is undergoing division. B, a filament in which the cells are 

 dividing to form four large swimming spores. C, a single swimming 

 spore drawn to a larger scale. D, a swimming spore beginning to 

 develop. E, a further stage of development. F and G smaller 

 swimming spores developing into new filament. 



rounding up into a sort of egg-shaped body. At the 

 smaller end the cytoplasm puts forth fine cilia — two in 

 number — and these new cells begin to move about inside 

 the old cell-wall. Presently they break through the wall 

 and escape into the water. They now begin a series of 

 rapid swimming movements and eventually each one con- 



