314 



THE STUDY OF ANIMAL LIFE CHAP. 



The hollow ball of cells almost always becomes dimpled 

 in or invaginated, as an india-rubber ball with a hole in it 

 might be pressed into a cup-like form. The dimpling is the 

 result of inequalities of growth. The two-layered sac 

 of cells which results is called a gastrula, and the cavity of 

 this sac becomes in the adult organism the digestive part 



of the food - canal. 

 Where there is no hol- 

 low ball of cells, but 

 some other result of 

 segmentation, the for- 

 mation of a gastrula is 

 not so obvious. Yet in 

 most cases some analo- 

 gous infolding is demon- 

 strable. 



In the hollow sac of 

 cells there are already 

 two layers. The outer, 

 which is called the ecto- 

 derm or epiblast, ^orms 

 in the adult the outer 

 skin, the nervous sys- 

 tem, and the most im- 

 portant parts of the 

 sense-organs. The in- 

 ner, which is called the 

 endoderm or hypoblast, 

 forms the lining of the 

 most important part of 

 the food-canal, and of 

 such appendages as 

 lungs, liver, and pan- 

 creas which are outgrowths from it. But in all animals 

 above the Sponges and Ccelenterates, a middle layer 

 appears between the other two. From this the meso- 

 derm or mesoblast the muscles, the internal skeleton, 

 the connective-tissue, etc., are formed. 



2. Differentiation. Development is the expression or 

 realisation of the inheritance. It is the making visible of 



FIG. 103. THE FORMATION OF THE TWO- 

 LAYERED GASTRULA FROM THE IN- 

 VAGINATION OF A HOLLOW SPHERE 

 OF CELLS. 



(From the Evolution of Sex ; after 

 Haeckel.) 



