14 INTRODUCTION. 
A series of vertical constrictions begins at the dorsal margin of each 
cceelomic 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 
vertebre, and so they were called protovertebre. 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 
myoccele. 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 vertebre 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 Cclom.—The parts of the cclom 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 coelomic 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 
