]S6 THE THIRD DAY. [CHAP. 



It will be remembered that tbe horizontal splitting 

 of the mesoblast into somatic and splanchnic layers 

 extends at first to the dorsal summit of the vertebral 

 plates, but after the formation of the somites the split 



Fio. 64. 



Transverse Section through the Trunk op a Duck 

 Embryo with about twenty-four Mesoblastio So- 

 mites. 

 am. amniou ; so. somatopleure ; sp. splanclinopleure ; wd. Wolf- 

 fian duct ; St. segmental tube ; ca.v. cardinal vein ; ms. 

 muscle-plate ; sp.ff. spinal ganglion ; sp.c. spinal cord ; ch. 

 notochord ; ao. aorta ; hy. hypoblast. 



between the somatic and splanchnic layers becomes to 

 a large extent obliterated, though in the anterior somites 



