THE EVOLUTION OF ANIMALS. 389 



3 . Practically all the higher animals pass next through 

 a stage in which the cells become differentiated into two 

 layers, more or less well defined and arranged in a kind 

 of double sac ectoderm on the outside and entoderm on 

 the inside (see Fig. 20, A4) . This is known as the gastrula. 

 Animals like the adult Hydra (Fig. 46) are really a kind 

 of permanent gastrula somewhat modified in form to 

 be sure, but a gastrula nevertheless. 



The facts thus far stated may be taken as suggesting 

 the following conclusions : 



1 . All organisms, even the highest, begin life at essen- 

 tially the same point; that is, as a single cell. This 

 similarity of individual origin indicates their fundamental 

 kinship and similarity of racial origin. 



2 . The development of all the forms above the protozoa 

 is parallel for at least a brief period ; that is, through the 

 morula and the blastula stages (Fig. 20, 3 and 4). This 

 parallelism of development added to the similarity of 

 origin points even more strongly to their kinship. 



3 . There is tendency for some forms to drop out of the 

 race and to become permanently fixed about certain of 

 these stages; as most protozoa at the single-celled stage, 

 Volvox at the morula stage, and sponges and hydra at 

 the gastrula stage. Others go on and introduce new 

 steps of differentiation before reaching their adult 

 development (Fig. 160: 5-9). 



386. Careful study of the later stages of development 

 of the higher animals gives us further illustrations of these 

 truths and enables us to state even more broadly the 

 principles deduced above. For example, we find that 

 insects have a parallel course of embryonic development 

 in which the great body of insects agree (Fig. 160). The 



