DOCTRINE OF EVOLUTION AND RELATED IDEAS 483 



into a simple mass of cells. This is known as the morula, which 

 is not unlike the adult, and highest stage, of such types as Eudor- 

 ina and Volvooc (colonial protozoa, Fig. 250, 2). 



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. 

 13 , A^) . This is known as the gastrula. Animals like the adult 

 Hydra (Fig. 81) are really a kind of permanent gastrula some- 

 what 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 simi- 

 larity 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. 13, 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. 250: 1-9). 



487. 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. 250). The same is true of the vertebrates. One in- 

 sect agrees with another insect in the mode of its development 

 more nearly up to the complete adult character than an insect 

 will agree with an echinoderm, or either with a vertebrate. 

 In a similar way, all vertebrates have a course of develop- 



