THE ANIMAL BODY 127 



vision, form on either side of the enteric pouch a linear 

 series, or band, of mesoderm cells. These MESODERM BANDS 

 gradually increase in size and spread out until finally they 

 unite above and below, that is encircle, the enteric pouch. 

 Thus they form a continuous mesoderm layer between 

 ectoderm and endoderm. Simultaneously with the growth 

 of the mesoderm bands to form a definite middle layer, a 

 linear series of spaces appears in each band which presages 

 the future segmentation of the worm's body. These cavities 

 increase in size and, when the bands unite around the enteric 

 pouch, the corresponding cavities of each band also become 

 continuous in the same regions. 



In this way the mesoderm becomes divided up into what 

 are essentially two cellular layers, an outer, or SOMATIC LAYER, 

 next to the ectoderm, and an inner, or SPLANCHNIC LAYER, in 

 contact with the endoderm. The space between these layers 

 of the mesoderm is the body cavity, or coelom. The coelom, 

 however, is not a continuous cavity from one end of the em- 

 bryo to the other, because the mesodermal cells which sepa- 

 rated the linear series of cavities in the respective mesodermal 

 bands persist. These cells form a regular series of connecting 

 sheets of tissue between the two mesoderm layers and thus 

 divide the body of the worm into a series of essentially similar 

 segments, or metameres, the limits of which are indicated on 

 the outside by a series of grooves which encircle the worm's 

 body. 



While these processes are transforming the two-layered 

 gastrula into an embryo composed of three primary layers, 

 and exhibiting metameric segmentation, coelom, etc., in 

 short, the 'tube within a tube' body-plan characteristic of 

 higher forms the embryo is gradually increasing in size and 

 elongating. The mouth, representing the blastopore, remains 

 at one end, which is therefore designated as anterior, while 



