THE SHAPES AND SIZES OF ANIMALS 129 



strong argument in favour of the primitive nature 

 of the delaminate gastrula ; while, as already 

 noticed, it tells strongly against the claims of its 

 rival, the invaginate gastrula, that it is at present 

 not possible to point out the progressive advantage 

 gained by the successive stages of gradual flattening 

 and gradual invagination through which the gastrula 

 stage is acquired. Indeed, we seem here driven to 

 suppose a much more rapid change to have occurred 

 than is commonly recognised as possible. Perhaps 

 the real explanation may be that the delaminate 

 gastrula is the older form historically, and that the 

 formation of a gastrula by invagination is merely an 

 embryological device to save time and facilitate the 

 course of individual development. 



The formation of the mouth, which in the de- 

 laminate gastrula is the final stage of development, 

 is an event of first-rate importance, both from the 

 morphological and physiological standpoints. Now 

 in the invaginate gastrula a mouth is, by the very 

 mode of development, present from the first com- 

 mencement of the process of invagination, and it 

 may be that the advantage gained by the early 

 formation of this important organ, which at once 

 obviates the necessity of the food having to traverse 

 the ectoderm cells in order to reach the digestive 

 layer or endoderm, it may be that this advantage 

 has led to the substitution in actual or individual 

 development of the invaginate for the historically 

 delaminate older type of gastrula formation. 



Turning from this somewhat lengthy digression 

 to our more immediate subject, we find that radial 



i 



