344 REPRODUCTION 



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 forma- 

 tion of which begins in the neck and proceeds caudad and cepha- 

 lad. They are sometimes called the protovertebrce. They 

 represent the primitive vertebrae. 



The body begins to assume shape and the fetal appendages 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 crescentic 

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

 as a simple furrow. This tucking-in finally surrounds 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 approach each other on its 

 ventral aspect and divide the yolk into two communicating cav- 

 ities. (See Figs. 102 and 103.) 



The layers of the blastoderm thus folded underneath the em- 

 bryo 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 through the vitelline 

 duct are the future alimentary canal and the yolk-sac, or umbil- 

 ical vesicle. It is to be noticed that the visceral plates embrace 

 both somatopleure and splanchnopleure, and that it is the 



