280 



GENERAL BIOLOGY 



the larger cells forming a plate "of clear columnar cells, and the small 

 cells spread out into a thin dome-shaped epithelium." 



The mesomeres lie toward the posterior end of the blastula and the 

 mesoblastic bands lie along the longitudinal axis of the worm, showing 

 the beginnings of bilateral symmetry. 



A gastrula is now formed by the invagination of the plate of large 

 cells, this invagination continuing until only a slit remains. This tiny 

 opening or slit is called the blastopore, while the cavity is the enteron. 



Fig. 172. Polygordius Appendiculatus. 



A, dorsal view, an, anus ; ct., cephalic 

 tentacles; h, head. B, trochosphere larva, an, Fig. 173. Nereis Pelagica.'^. (After Oersted), 



anus ; e, eye-spot ; m., mouth. C and D, stages 

 in development of trochosphere into the worm. 

 pnp, pronephridium. (From Bourne, after 

 Fraipont). 



There are now three germ-layers. The mid-layer or mesoderm al- 

 ready began forming before gastrulation. 



The large clear cells which invaginated have become the inner lining 

 of the enteron and form the entoderm; the outer portion is ectoderm, 

 while the mesoderm is made up of the two mesoblastic bands which lie 

 between ectoderm and entoderm. 



As the earthworm is to be our example of the coelomates it is of 

 value here to observe how the coelom is formed. 



The mesoderm separates into the two layers on each side of the 

 body. A cavity forms between these layers. This cavity is the coelom. 

 The outer portion of the divided mesoderm is called the somatopleure 

 ( ), the inner layer the splanchnopleure ( ). 



The muscles of the body-wall are formed from the somatopleure, 

 while the splanchnopleure forms the muscles of the alimentary canal. 

 After the germ-layers are formed, the embryo elongates, the anterior- 

 posterior axis passing through the blastopore. There are various in- 

 pushings from the ectoderm which become the elements of the nervous 

 system. Such beginning cells are called neuroblasts if they form nerves. 



