1 8 HISTOLOGY 



widely separated cells are not assembled to form a definite region in 

 the colony. 



Second Division: Grantia sp. In Grantia and other sponges we meet 

 with a triploblastic colony ; by this we mean a cell aggregate composed 

 of three layers of cells. The outer layer of cells forms a thin layer of 

 nucleated cytoplasm (Fig. 16, ect.). The inner layer is composed chiefly 

 of oval cells supplied at their free ends with a bell-shaped collar and a 

 slender flagellum (Fig. 16, end.}. Between them lies a layer of greatly 

 branched, anastomosing cells. These cells bear rounded nuclei (Fig. 16, 

 mes.}. We observe here that the cells modified to perform the function 

 of protection are assigned to a particular region of the cell-colony and 

 that they form a continuous outer layer, the ectoderm (ect.). Likewise 

 the cells performing the function of alimentation are assembled and 

 relegated to particular regions to form part of the endoderm (end.). 

 Finally, between these two tissues we note the third tissue made up of 

 cells looking chiefly after the mechanical support of the colony and unit- 

 ing to form a loose tissue the mesoderm (mes.). 



LITERATURE 



Read general articles and parts of such books as "The Foundations of Zoology," by 

 W. K. Brooks. 



