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 (Iigs. 
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 
Fic 107. — Transverse section through the twenty-ninth somite of a 29s 
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 29s embryo. In the twenty-ninth somite 
