l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



of the coelom, however, it might pass for the ancestor of a flatworm, 

 for even in the Platyhelminthes there is a teloblastic proHferation of 

 cells that appear to correspond with the mesoderm cells of the anne- 

 lids, though the cells thus produced soon disperse and become a part 

 of the parenchyma. It is in the development and elaboration of the 

 mesoderm, or teloblastic mesoblast, therefore, that the Coelomata 

 depart from the Platyhelminthes. Segmentation is a feature super- 

 imposed upon the mesoderm in the Annelida as a result of body 

 metamerism, in which apparently the ingrowth of the septal muscles 

 plays an important part. 



The mesoderm of the adult annelid or arthropod gives rise to a 

 large variety of tissues and organs, but most of the specialized deriva- 

 tives of the mesoderm are formed in the secondary segments of the 

 adult animal. The principal products of the primary mesoderm are 

 muscle and connective tissues, and an epithelial lining of the coelomic 

 cavities. 



According to E. Meyer (1901), the mesodermal myoblasts of the 

 polychaete larva are not recognizable as such until the mesoderm 

 bands have become broken up into segmental sections, and the trans- 

 formation of the myoblasts into functional muscle fibers is not evident 

 until after the appearance of the coelomic cavities. The myoblasts of 

 each mesodermal segment, Meyer says, consist of four large cells on 

 each side, two dorsal and two ventral, lying along the lines of the 

 larval longitudinal muscles of mesenchymatic origin already present. 

 The mesoderm fibers finally replace the mesenchyme fibers and be- 

 come the definitive longitudinal muscles of the worm. The coelomic 

 myoblasts, Meyer shows, are true epithelial muscle cells that form 

 muscle processes from their outer surfaces, while the plasmatic bodies 

 of the cells retain their places for some time in the coelomic walls. 

 The parts of the coelomic walls not involved in muscle formation be- 

 come thinner, and finally transform into typical peritoneal epithelium. 



The important part that the mesoderm plays in the development of 

 the coelomate animals is entirely clear ; but what the mesoderm be- 

 comes in the course of evolution does not explain what it was in its 

 beginning. Most of the theories that have been proposed to account 

 for the primitive mesoderm, it will be found, are attempts to explain 

 the functional nature of the coelomic sacs rather than the origin of 

 the mesoderm itself. 



The theory most widely accepted at one time as to the origin of 

 the mesoderm is the enterocoele theory (Hertwigs, 1882, Sedgwick, 

 1884), by which the coelomic sacs are explained as diverticula of the 

 archenteron. In some animals the coelomic sacs are thus formed in 



