FROM TWELVE TO THIRTY-SIX SOMITES 



185 



52, (see Keibel and Abraham, Normaltafeln). In an eight-day chick 

 the number of somites is again about 42, including the four fused with 

 the skull. Thus the ten somites formed last are again lost. This points 

 towards a long-tailed ancestry for birds. 



^'Although the somites have the same fundamental structure 

 in all parts of the body, they differ greatly in many respects" 

 (Williams). It is not, however, our purpose to consider the indi- 

 vidual characters of each pair of somites, but rather the relations 

 common to all. 



Each somite is composed of an epithelial wall of high, columnar 

 cells, enclosing a core of cells that nearly fills the cavity (Figs. 

 112, 113, etc.). From each somite there arise three parts of 

 fundamental significance, viz., the sclerotome, the muscle plate, 

 and the cutis plate (dermatome), the primordium of the axial 



/).€/: 



S 23.-^^1 



■^^3t^ 



Coe/ 





^^^?^^-*,. ^i? 



■Som'pL 



N'c/i. 



IVD. 



//epA 



^; 



JiO- 



Coel. 



^/'^y 



Fig. 107. — Transverse section through the last somite of a 29 s embryo. 



n. Cr., Neural crest. Neph., Nephrotome. W. D., Wolffian duct. Other 

 abbreviations as before. 



skeleton, the voluntary muscles (excepting those of the head), 

 and derma respectively. The manner of origin of these parts 

 may be studied fully in an embryo of 25 to 30 somites, by com- 

 paring the most posterior somites, in which the process is begin- 

 ning, with somites of intermediate and anterior positions in the 

 series, which show successively later stages. 



Figs. 107, 108, 109, and 110 represent transverse sections 

 through the twenty-ninth, twenty-sixth, twentieth, and seven- 

 teenth somites of a 29 s embryo. In the twenty-ninth somite 



