196 



The Study of Animal Life part m 



H 



food- canal. Where there is no hollow ball of cells, but 

 some other result of segmentation, the formation of a gastrula 



is not so obvious. Yet 

 in most cases some 

 analogous infolding is 

 demonstrable. ■ 



In the hollow sac 

 of cells there are 

 already two layers. 

 The outer, which is 

 called the ectoderm 

 or epiblast, forms in 

 the adult the outer 

 skin, the nervous 

 system, and the most 

 important parts of the 

 sense - organs. The 

 inner, which is called 

 the endoderm or hypo- 

 blast, forms the lining 

 of the most import- 

 ant part of the food- 

 canal, and of such 

 appendages as lungs, 

 liver, and pancreas 

 which are outgrowths 

 from it. But in all 

 animals above the 

 Sponges and Coelenterates, a middle layer appears between 

 the other two. From this — the mesoderm or mesoblast — 

 the muscles, the internal skeleton, the connective-tissue, etc., 

 are formed. 



9. Some Generalisations. — {a) The '^ Ovum -Theory." 

 To realise that almost every organism from the sponge to 

 the highest begins its life as a fertilised egg-cell, and is 

 built up by the division and arrangement, layering and fold- 

 ing of cells, should not lessen, but should greatly enhance, 

 the wonder with which we look upon life. If the end 

 of this constantly repeated process of development be 



Fig. 37.— The formation of the two-layered gas- 

 trula from the invagination of a hollow sphere 

 of cells. (From the Evolution of Sex ; after 

 Haeckel.) 



