126 THE SHAPES AND SIZES OF ANIMALS 



with the exterior, and the embryo becomes a 

 gastrula. 



It is by no means easy to determine which of 

 these two forms is the more primitive. The in- 

 vaginate gastrula is much more widely distributed 

 in the animal kingdom, occurring, as we have seen, 

 in all the large groups, while the delaminate gas- 

 trula is much more restricted. On the other hand 

 it is not easy to see how the invaginate gastrula 

 first came into existence, for it is by no means 

 clear what advantage a spherical blastula-like 

 animal gets by becoming flattened on one side, 

 or what further advantage is conferred by a very 

 slight depression or cupping of this flattened sur- 

 face. Such a depression is useful enough after it 

 has reached such proportions as to give rise to a 

 sac-like cavity suitable for the reception and diges- 

 tion of food, but the early stages of its formation 

 are useless, and could not have been preserved for 

 such purpose. In the case of the delaminate' 

 gastrula however there is no such difficulty, and it 

 is possible to construct a hypothetical series of 

 forms which may well represent the ancestral 

 series in the pedigree of the gastrula, each step 

 marking a distinct advance in organisation, and 

 being a sufficiently definite improvement to justify 

 its perpetuation ; and the whole series correspond- 

 ing to the successive stages of development of the 

 delaminate gastrula of the present day. 



Starting with the blastula stage, a hollow ball 

 whose wall is but one cell thick, we note that the 

 inner and outer ends of each of the cells are 



