FROM TWELVE TO THIRTY-SIX SOMITES 



183 



VI. History of the ^Mesoderm 



The history of the extra-embryonic mesoderm is considered 

 sufficiently in the first part of this chapter. The history of the 

 embryonic mesoderm will be considered under the following 

 heads: (1) Somites, (2) Intermediate Cell-mass, (3) Vascular 

 System, (4) Lateral Plate and Body-Cavity, (5) Mesoblast of 

 the Head. 



-"-■'f^^' tr.Am 





Fig. lOo. — Embryo of about 27 somites drawn in alcohol by re- 

 flected light; upper side, x 10. 

 Am., Amnion, ot., Otocyst. t. F. Am., Tail fold of amnion. 



(1) Somites. The rate of formation of the somites from the 

 segmental plate and their number at different times is given in 

 the normal table of embryos (p. 68), and may be seen in various 

 figures of entire embryos. The formation of new somites con- 

 tinues after the end of the period discussed in this chapter, up 

 to about the sixth day. Each somite has a definite value in the 

 developmental history. 



