STRUCTURAL CHARAC1ERS OF ANIMAL ORGANISMS. 



41 



be placed in a kingdom by themselves (as has been proposed) 

 apart from the vegetable and animal kingdoms. 



The group of these organisms which most resemble animals, 

 is called Protozoa, and is divided from other animal forms by 

 the manner of development of the ovum of the latter, which 

 differentiates by division into cells. This group is called the 

 Metazoa. In the Protozoa the ovum never divides, the animal 

 always remaining a single cell. 



On the contrary, the ovum of the Metazoa changes its charac- 

 ters during its development. At first possessing a stage common 

 to both divisions, viz., a single cell, it soon passes through rapid 

 stages of cell proliferation, and is converted into a multiple mass, 

 the mulberry stage or Morula. 



The cells forming this Morula stage work toward the periphery 

 of the mass, where they tend to arrange themselves in two layers, 

 at the same time forming a cavity in the 

 centre. This is known as the Gasirula stage. 

 Following, then, this cell multiplication or 

 quantitative differentiation, we find a quali- 

 tative differentiation of the cells, by which 

 certain groups of cells assume special peculi- 

 arities, fitting them for some specific duty. 



Thus we arrive at the production of spe- 

 cial textures and organs such as are met 

 with in the higher animals, and which are 

 necessary for the efficient discharge of the 

 various functions carried on during their 

 lives. The division of the original mass of Diagram showing the 

 indifferent cells into two layers of special first differentiation of the 



11 .1 / i .. -i. organism into an external 



cells is the first step toward tissue differen- and internal i aye r. (> 

 tiation, and in some animals is the only one Mouth, (t) enteric cavity, 



FIG. 8. 



arrived at in their entire life history through- 



out which they remain a simple sack made 



up of an external layer, Ectoderm, and an internal layer, Endo- 



derm. 



The groups or layers of cells forming the outer and inner layers 

 of this stage of development, not only form the primitive tissues, 

 4 



