Animal Causes. 213 



as in everything else, just comes without any foresight 

 whatever. Everything is undoubtedly produced by 

 inviolable law, but no law working towards a pre- 

 meditated end. There is thus a delightful sense of 

 expectancy, wonder, newness, and anticipation in 

 universal operations. Everything is possible, but only 

 the actual happens. 



The sequence of structural differentiation proceeds 

 somewhat on the following lines in all animals. 



All animal cells are alike in appearance, and con- 

 sist of a cell-wall enclosing protoplasmic elements in 

 solution. After impregnation a cell attracts and 

 aggregates protoplasmic components similar to itself 

 from its environments. This action constitutes growth. 

 After attaining maturity it divides similarly to the 

 vegetal cell. 



Continued fission or cell-division results in an 

 aggregate of cells or cell-aggregate ; then follows the 

 formation of an external layer of small polyhedral 

 cells round the cell-aggregate, called the blastoderm, 

 the interior cavity being filled with fluid. Such an 

 organism is called a planula. Next, invagination 

 occurs, or the pushing in of one side of the spheroid 

 planula, until from a cup it grows into a double-walled 

 sac with an opening ; in other words, a mouth and 

 stomach are formed. This constitutes a gastrula, the 

 simplest ancestral form of the metazoa, that is, all 

 animals above protozoa. 



The outer layer or epidermis of this gastrula is 

 called the epiblast, and the inner the hypoblast. But 



