1 6 INTRODUCTION 



from the splanchnic wall cells are budded, which, like those from 

 the myotomes, form part of the skeletogenous tissue. Since both 

 the excretory portions of the middle plate and the skeletogenous 

 parts of both myotome and middle plate are metameric in origin 

 and retain their metamerism for some time, they are called respect- 

 ively nephrotomes (fig. 9, neph) and sclerotomes (fig. -8, ck), the 

 cavities in the nephrotomes being the nephrocoeles. The history 

 of the skeletal and excretory systems will be given later. 



The Coelomic Cavities. — As has been described above, the coelom 

 in each mesothelial pouch becomes divided into a series of myocoeles 

 and nephrocoeles, connected with a larger cavity, the metacoele, 

 in the hypomeral or lateral-plate region. With the separation of 

 the myotomes the myocoeles lose their connexion with the other 

 cavities, and when the somatic walls of the myotomes break up 

 into mesenchyme, these cavities are lost. The nephrocoeles form 

 the cavities in the tubules of the excretory system, and in the lower 

 vertebrates these never separate from the metacoele, but open- 

 ings, the nephrostomes, connect the tubules with the larger cavity. 

 The history and modifications of these are traced in the section on 

 the excretory organs. 



The metacoele is at first undivided and the cavities of the two 

 sides are distinct. With increasing age the hypomeres of the two 

 sides gradually extend downward between ectoderm and entoderm 

 until their lower margins meet, ventral to the digesive tract. In 

 this way the latter becomes surrounded by a pair of metacoelic 

 cavities. Each cavity is bounded by epithelium, the tunica serosa, 

 the outer or somatic portion of which is turned toward the ectoderm 

 while the splanchnic portion adjoins the entoderm of the digestive 

 tract. Later, when the myotomes extend into this region (fig. lo) 

 they, with the invading mesenchyme, unite the ectoderm and somatic 

 serosa into the thick outer body wall, the somatopleure, while a 

 similar migration of mesenchyme between serosa and entoderm unites 

 these into the wall of the alimentary canal, the splanchnopleure. 



Mesenteries. — As has just been stated, the ventral margins of the 

 two coelomic pouches meet below the digestive tract, thus forming a 

 double membrane running lengthwise of the body and binding the 

 alimentary canal to the ventral body wall. This is the ventral 

 mesentery (fig. 9, vmes). At the same time the splanchnic walls of 

 the two cavities meet just above the entoderm and between it and 

 the notochord, forming a dorsal mesentery which ties the digestive 



