MESODERM AND CCELOME 



583 



derm and endoderm, or both endoderm and mesoderm may 

 be differentiated at the same time from the lower layer- 

 cells or yolk-cells (e.g. frog) ; or, finally, it may arise in 

 all these ways (e.g. fowl, rabbit). The ccelome is formed 



sp. c 



so. 



om. 



FIG. 158. Transverse section through the trunk ot a duck-embryo with about 

 24 mesodermic segments, for comparison with Fig. 157 : the peripheral part of 

 the blastoderm is not shown. 



am. developing amnion ; ao. aorta ; ca.v. cardinal vein ; ch. notochord ; hy. 

 endoderm, which overlies the unsegmented yolk ; ms. muscle-plate of meso- 

 dermal segment ; sc. somatopleure, and sp. splanchnopleure, with the coelome 

 between them ; sp.c. spinal cord ; sp.g. spinal ganglion of neural ridge ; st. 

 mesonephric tubule ; -wd. mesonephric (Wolffian) duct. (From Balfour's 

 Embryology.} 



by a split taking place in the mesoderm on either side 

 (Fig. 65, .157, and 158), the split gradually extending with 

 the extension of the mesoderm between the ectoderm and 



