186 THE THIRD DAY. [CKAP. 



It will be remembered that the 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 



Fig. 64. 



'F9 SI 



Transverse Section through the Truntc of a Duck 

 Embryo with about twentt-four Mesoblastic So- 

 mites. 

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

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

 muscle-plate ; sp.g. spinal ganglion ; sp.o. 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 



