324 ORGANIC GROWTH sec. 



and these became synchronous on all sides, and thereby the 

 locomotion of the whole forwards or backwards took place. 



Those cells which were situated at the pole of the blas- 

 tula must have specially developed their power of food-incep- 

 tion, because they had most opportunities of exercising it : 

 they came first and most frequently into contact with food. 

 As several neiohbourino- cells thus devoted themselves more 



o o 



and more exclusively to food-inception, the first step was 

 taken, functionally, in the evolution of the first common 

 organ, of the digestive endoderm, and also in the formation of 

 an invagination of one pole of the blastula, which gave rise 

 to a second common organ, the primitive gut. Every increase 

 in the degree of this invagination was an advantage to the 

 nutrition of the whole, for more food could be taken up in it. 

 In this respect, therefore, selection may have been actively at 

 work. Thus arose the gastrula, an organism of the form of 

 an egg-cup, with a wall of two layers of cells lying one over 

 the other, of which the inner is derived from the invagination 

 of an original single layer. 



With this invagination we get at once two chief or primi- 

 tive organs of the multicellular animals, the ectoderm and 

 endoderm. While the latter serves only for the reception of 

 food, the former has functions of three kinds : — 



(1) The protection of the body on the outside, as its cover- 

 ing ; (2) to act as the organ of relation, instead of a nervous 

 system, to place the body in communication with the outer 

 world; (3) locomotion. 



With regard to the latter, it is to be observed that it was 

 probably s, definitely-directed activity, namely, locomotion in 

 a definite direction, which led to the development of per- 

 manent flagella on the cells in place of the irregular, variable 

 pseudopodia. And this modification must have taken place 

 before the completion of the endoderm, for the latter itself 

 still bears flagellate cells, while at the same time it can also 



