14 INTRODUCTION. 



A series of vertical constrictions begins at the dorsal margin of each 

 ccelomic pouch and cuts down through epimere and mesomere, so 

 that the whole may be compared to a glove with a large number of 

 fingers extending from its upper surface, each finger being hollow, and 

 all of the cavities connecting with that in the hypomere (palm). This 

 process begins at front and gradually extends backward. Viewed 

 from above in the transparent embryo, each of these fingers appears 

 like a square box and early students thought that they gave rise to the 

 vertebrae, and so they were called protovertebrae. Next, the dorsal 

 part of each of these fingers is cut off from the rest, along the line 

 between mesomere and epimere., thus forming a series of hollow cubes, 

 known as myotomes, each with a part of the ccelom in its interior, the 

 myocoele. After the separation from the rest each myotome grows 

 upward and to a greater extent downward, insinuating itself between 

 the ectoderm and the somatic wall of the hypomere (fig. 9, in the 

 direction of the arrows). From these myotomes the body (somatic) 

 musculature arises. 



From the medial mesomeral part of the fingers arises the mesen- 

 chyme that gives origin to the vertebras while the rest furnishes the 

 material for the excretory organs. From their origin both of these 

 are metameric at first, the skeletogenous parts being called scler- 

 otomes, the excretory parts, nephrotomes (fig. 8, mm, st). The 

 history of both will be followed in their proper places. 



The Ccelom. The parts of the ccelom in the myotomes soon 

 disappears, that in the nephrotomes, of inconsiderable size, forms 

 the lumina of the excretory ducts. That in the hypomere (fig. 9, c) 

 forms the large body cavity (peritoneal cavity) surrounding the 

 chief viscera, and the smaller one (pericardial) around the heart. 

 In surrounding the archenteron the walls of the two ccelomic cavities, 

 which at first are separate, tend to meet above and below the entoderm, 

 so that there is in both regions a thin membrane supporting the digest- 

 ive tract above and below. Such supports are collectively called 

 mesenteries. Usually that below (v mes) largely disappears, but the 

 dorsal (d mes) one persists more or less completely. At first these 

 mesenteries are merely double membranes of mesothelium, but soon 

 mesenchyme grows in between them and extends around the digestive 

 tract, so that mesothelium and entoderm are bound together by the 

 invading tissue. In a similar way the somatic wall of the ccelom is 

 bound to the muscles arising from the myotomes and these in turn to 



