122 THE SHAPES AND SIZES OF ANIMALS 



towards the surface, get flattened more and more 

 until ultimately they become converted into the 

 scale-like cells. Thus, each scale-like or pavement 

 epithelial cell is in the first instance, in its earliest 

 stage of existence, a spherical cell. So also with 

 the columnar cells of the stomach or intestine. 

 Each such cell is formed in the deeper layers of 

 the epithelium as a spherical cell, and gradually 

 becomes elongated into a columnar cell as it 

 approaches the surface. The spherical cell is 

 therefore the link connecting the scale-like and the 

 columnar cells; an indifferent or primitve form, 

 from which either of the more modified forms may 

 be derived, and which is really the earliest stage in 

 the developmental history of both. It would be 

 easy to multiply instances of this kind, but I have 

 said enough to show that there is really strong 

 ground for .holding that the spherical form is to be 

 regarded as a primitive one ; perhaps as the most 

 primitive form met with amongst animals. 



The truly spherical shape, in which all parts of 

 the surface are alike and of equal value, is only 

 seen in the unicellular animals, or Protozoa, and in 

 the individual cells of higher animals. The early 

 embryonic phases of many of the higher animals, 

 known as Morula and Blastula, are also spherical, 

 the former being a solid heap of spherical and 

 polygonal cells resulting from the repeated division 

 of the fertilised ovum ; while the blastula is a later 

 stage, having the form of a hollow ball with a wall 

 composed of a single layer of cells surrounding 

 a cavity filled with fluid. Neither morula nor 



