THE SHAPES AND SIZES OF ANIMALS 121 



spherical shape is not merely the simplest which an 

 animal can offer, but is also the most primitive. 

 In evidence of this we may quote the spherical 

 shape of the ovum, which is characteristic of all the 

 higher animals as regards the earliest stage of its 

 formation, and of most as regards its fully matured 

 condition. If we are right in regarding the 

 embryological development of an animal as a re- 

 capitulation of its ancestral history, then the earliest 

 developmental condition i.e., the ovum, or egg 

 must represent the most primitive ancestral phase, 

 and the significance of the spherical shape so 

 characteristic of the earliest condition becomes at 

 once apparent. 



A further argument in favour of the primitive 

 nature of the spherical form may be drawn from 

 the development of the more modified forms of 

 cells, even in adult animals. Thus, the shapes of 

 the epithelial cells vary greatly, according to the 

 part from which they are taken. Cells from the 

 surface-layer of the epidermis, such as those lining 

 the mouth or covering the hand, are thin scales 

 fitted together edge to edge, and with their flat 

 surfaces parallel to the surface which they cover. 

 Cells from the epithelial lining of the stomach 

 or intestine on the other hand, are columnar or 

 rod-like in form, being placed side by side with 

 their long axes vertical to the surface they clothe. 

 Yet a section through the epidermis shows that 

 its deepest layer consists of spherical cells, which 

 gradually approach the surface as those lying over 

 them get rubbed away, and which, as they move 



