FROM TWELVE TO THIRTY-SIX SOMITES 183 
VI. History or 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. 
WS 
Bytes 
Oe 
Fig. 105. — Embryo of about 27 somites drawn in aleohol by re- 
flected light; upper side. x 10. 
Am., Amnion. ot., Otoecyst. 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 
