186 



THE THIRD DAY. 



[CHAP. 



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. 



am. 



so. 



TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK 

 EMBRYO WITH ABOUT TWENTY-FOUR MESOBLASTIC SO- 

 MITES. 



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

 nan duct ; st. segmental tube ; ca.v. cardinal vein ; ms. 

 muscle -plate ; sp.g. spinal ganglion ; sp.c. spinal cord ; ch, 

 notochord ; ao. aorta ; Jiy. hypoblast. 



between the somatic and splanchnic layers becomes to 

 a large extent obliterated, though in the anterior somites 



