CLEAVAGE 327 



ism, whether it is a unicellular form or whether it is a higher 

 plant or animal. Only a few typical cases will be outlined. 



704. (i) When the cell has divided the daughter cells sepa- 

 rate completely. On future divisions the same thing occurs 

 and all the cells remain alike. This type includes the free 

 protozoa and many unicellular plants. Sometimes a number 

 of divisions occur before the cells separate, so that a group of 

 eight, sixteen, etc., cells are produced. These then all become 

 free at once and become independent organisms. 



705. (2) The cells divide, but are held together by a gelatin- 

 ous matter which they secrete (Fig. 60) ; or by their cell walls 

 (Figs. 182 and 212), or by a connecting bridge which develops 

 into a branching stalk. These are colonies, and the arrange- 

 ment of the individuals in the colony depends largely on whether 

 the planes of division are always parallel, forming filaments 

 (Figs. 182 and 212), or at right angles in two planes, form- 

 ing plates, or at right angles in three planes, forming cubical 

 masses. (Fig. 60.) 



706. (3) The cells divide structurally, but remain in func- 

 tional unity. The resulting entity is not a colony, but still re- 

 mains a single organism. Because of cell division the nuclear 

 matter is distributed throughout the body of the organism, and 

 the size of the body is not limited by the limited distance 

 through which the nucleus acts on the cytoplasm, as would 

 probably be the case if there were but a single central nucleus. 

 The division of the body into cell units also permits differen- 

 tiation to an unlimited degree, and with it division of labor 

 and perfection of function. (See pp. 136, 338.) 



707. Under this head two types of development occur which 

 we may characterize as evolving and involving. In the former 

 the cell mass is solid and leads to the diffuse, plant type of 

 organization, while in the latter the cells are arranged in layers 

 enclosing cavities, and through it the compact animal type of 

 organization is attained. (See pp. 328, 331 ff.) This apparent 



