BODY CAVITY 345 



and carry with them chordal folds of the entoderm. When 

 the curling edges have joined to form a solid cylinder of 

 cells, the chordal folds unite over the ventral surface of the 

 cylinder. Figures 99 and 100 illustrate these facts. The 

 notochord is in the line of the future vertebral bodies, but it 

 is not developed into any adult structure. 



Somites. These are masses of cells developed from the 

 axial plates of the mesoderm, lying parallel with and on each 

 side of the notochord. (Fig. 100.) They are in segments, 

 the formation of which begins in the neck and proceeds 

 caudad and cephalad. They are sometimes called the pro- 

 tovertebra. They represent the primitive vertebrae. 



The body begins to assume shape and the fetal append- 

 ages to be developed at the same time. The latter are for 

 the protection and nutrition of the embryo. The essential 

 parts of a vertebrate are a vertebral column with a neural 

 canal above and a body cavity below it. The body cavity 

 contains the alimentary canal. The somites representing the 

 vertebral column and the formation of the neural canal have 

 been noticed. 



Body Cavity. At first the embryo, as represented by the 

 embryonal area, is on a level with the remaining surface of 

 the blastoderm. Soon, however, there appears, marking the 

 head of the embryo and with its concavity backward, a cres- 

 centic folding in of the blastodermic wall. It is evident on 

 the surface as a simple furrow. This tucking-in finally sur- 

 rounds the whole embryonal area, and the surface fissure, 

 now oval, becomes deeper and deeper, until those portions 

 of the wall which are being tucked under the embryo ap- 

 proach each other on its ventral aspect and divide the yolk 

 into two communicating cavities. (See Figs. 102 and 103.) 



The layers of the blastoderm thus folded underneath the 

 embryo are the visceral plates. They form the boundaries 

 of a cavity which still communicates in front, at the site of 

 the future umbilicus, with the yolk-sac. This narrow canal 

 is the vitelline duct, and the two cavities communicating 



