14: THE LOWEST ANIMALS EVEN SIMPLER THAN A CELL. 



FIG. 10. 



^mMtm?m 



Branchial cartilage from a Tadpole, showing cells in various phases, a, four cells, 

 formed by the division and subdivision of one parent cell; I), pair of cells in appo- 

 sition; c, nuclei; d, cavity with three cells. Magnified about 350 diameters. 



inent made above is true, that animals are made tip of 

 tissues, there are some kinds of animals so simple that a 

 single cell constitutes the whole animal. And the same is 

 true of some kinds of plants. That is, there are unicellu- 

 lar animals, and unicellular plants * (Fig. 11). 



Nay, we must go a step farther, and say that microscop- 

 ists have shown that the simplest or lowest animals and 

 plants are merely nucleated particles of protoplasm, these 

 particles not even having a cell-wall that is, they are not 

 inclosed in a sac; and that some kinds are so simple that 

 the particles are not even nucleated. 



And further, it is pretty clearly demonstrated that all 

 animals, even the highest forms, begin their embryonic 

 existence as mere particles or aggregated particles of pro- 



* Botanists describe the vegetable cell as consisting of first, a membrane 

 or permanent cell-wall ; second, of a delicate mucilaginous film lining this 

 wall, and called by Mohl the primordial utricle ; third, of a more or less soft 

 or gelatinous body in the center of the cell, and called the nttcleus ; and 

 fourth, of a viscid liquid called protoplasm, which often has an abundance 



