THE SHAPES AND SIZES OF ANIMALS 127 



exposed to very different environment. The outer 

 ends being on the surface of the blastula can come 

 in contact with and gain cognisance of the outer 

 world, while the inner ends facing towards the 

 blastula cavity can have no direct contact or con- 

 cern, except with bodies that have passed inwards 

 through the outer parts of the cells. Hence 

 merely as a consequence of this arrangement, 

 physiological differentiation will be set up between 

 the outer and inner ends of each cell ; the outer 

 parts of the cells will become the seats of sensation 

 and of locomotive activity, while the inner ends, 

 freed from these functions, apply themselves to 

 other purposes and become specially nutritive or 

 digestive in function. The next stage is a simple 

 one. The differences between the outer and inner 

 ends of each cell once established will tend to 

 increase as each part of the cell learns to discharge 

 its special functions more efficiently : a mechanical 

 separation of the two parts of the cell is but a slight 

 further differentiation ; each cell dividing into two 

 an outer cell, sensitive and locomotive and probably 

 respiratory in function, and an inner cell specially 

 digestive in purpose. 



So far we have supposed the food to consist of 

 small particles captured by the outer parts of the 

 cell, or the surface cells where two layers are 

 established, and passed inwards to the inner parts, 

 or the inner cells, to be digested. The act of di- 

 gestion is still intra-cellular i.e., is effected entirely 

 within the substance of the cells just as in an 

 Amoeba. If now we suppose particles of larger 



