128 FOUNDATIONS OF BIOLOGY 



growth is chiefly in the opposite direction or toward the pos- 

 terior. At this end (the blind end of the enteric pouch formed 

 at gastrulation) an opening to the exterior, the anus, is formed 

 so that the enteric pouch now communicates with the ex- 

 terior at both ends and becomes the alimentary canal. Thus 

 antero-posterior differentiation is clearly established. 



A cross section perpendicular to the main axis of the devel- 

 oping worm at this stage presents the appearance of a circle 

 within a circle. The smaller circle surrounds the enteric 

 cavity and is the wall of the alimentary canal. It is separated 

 by a space, the coelom, from the larger circle, or body wall. 

 Moreover, each of these circles is composed of two tissue 

 layers: the alimentary canal, formed internally of endoderm 

 and externally of mesoderm; and the body wall, internally 

 of mesoderm and externally of ectoderm. Thus the coelomic 

 cavity is entirely enclosed by mesoderm. 



It is from these four layers of cells (ectoderm, somatic and 

 splanchnic mesoderm, and endoderm) that all of the tissues 

 and organs of the adult worm arise through thickenings, 

 foldings, outgrowths, etc. For example, the nervous system 

 is formed by the ingrowth of a thickened region of the ecto- 

 derm; the blood vascular system develops by a specialization 

 of cells throughout the mesoderm; while the reproductive 

 system first appears as thickenings of the somatic mesoderm 

 which, as development proceeds, becomes largely separated 

 from it as independent organs in the coelom. In general, it 

 may be said that in all the higher animals the ectoderm forms 

 the outer skin and nervous system; the endoderm supplies 

 the lining membrane of the major part of the alimentary 

 tract; while the mesoderm contributes muscles, blood vessels, 

 reproductive organs, and the membrane lining the coelom. 

 This similarity in origin of the organ systems throughout 

 the animal series above Hydra and its allies is of the highest 



