194 THE EVOLUTION OF MAN. 



naked eye, or, at most, under favourable circumstances, it 

 is seen as a tiny speck, usually ^o — iu> or a ^ mos ^ i — I 

 millimetre in diameter ; it is hardly ever more. In form 

 the body of the Gastrula is usually cup-like ; sometimes it 

 is rather egg-shaped, sometimes rather ellipsoid or fusiform ; 

 in other cases it is more hemispherical, or almost spherical ; 

 and again in others, longer or almost cylindrical. The 

 geometric outline of the body is highly characteristic; it 

 is marked by a single axis with two differing poles. This 

 axis is the main, or longitudinal axis of the future animal 

 body ; one pole is the mouth, or oral pole ; the opposite is 

 the aboral pole. This outline with one axis distinguishes 

 the Gastrula very essentially from the globular Blastula 

 and Morula, in which all the axes of the body are similar. 63 



I shall call the central cavity of the Gastrula-body the 

 primitive intestine (protogaster), and its opening the pri- 

 mitive mouth (protostoma). For this cavity is the original 

 nutritive, or intestinal cavity of the body, and this opening 

 originally served to admit food into the" body. It is true 

 that at a later period the primitive intestine and the 

 primitive mouth appear very different in the different 

 tribes of animals. This is especially true of Vertebrates, 

 in which only the middle portion of the later-formed in- 

 testinal canal proceeds from the primitive intestine; and 

 in which the later mouth-opening is a formation entirely 

 independent of the primitive mouth, which closes. It is, 

 therefore, necessary to distinguish clearly between the 

 primitive mouth and intestine of the Gastrula on the one 

 hand, and the later-formed intestine and mouth of the 

 developed Vertebrate on the other. 64 



The two cellular layers which surround the cavity of 



