FROM TWELVE TO THIRTY-SIX SOMITES 185 



26 to 32. Leg somites. 



33 to 35. Region of cloaca. 



36 to 42. Caudal. 



More somites are formed later, the maximum number recorded being 

 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. 



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 



n.Cr. 



Som'p/. 



Coet. 



FIG 107. Transverse section through the twenty-ninth somite of a 29 s 

 embryo. *V\-WV- 



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 



