Charles Eussell Bardeen and Warren Harmon Lewis 



17 



median layer of the myotomes, the formation of the spinal ganglia, and 

 the appearance of the ventral nerve roots. This is shown in Fig. 18, 

 taken throngh the leg region of Emhryo LXXVI. 



Fig. 18. Cross section tbrougli tlie lumbar region of Embryo LXXVI. x 55 d. 



Fig. 19, taken through the leg region of Embryo CXLVIII, shows an 

 older stage in which the limb-bud is considerably more advanced in 

 development. The spinal ganglia are distinct. Formation of muscle 

 cells has begun on the median surface of the myotomes. 





Fig. 19. Cross section tbrough the lumbar region of Embryo CXLVIII. x 55 d. 



The Wolffian ridge and limb-buds, which appear during the third 

 week, as shown in Figs. 2, 3 and 4, consist, therefore, at the end of this 

 period, merely of a mass of mesenchyme which intervenes between the 

 coelom and the ectoderm lateral to the axis of the body. This mesen- 

 chyme contains a vascular plexus. Along the free edge of the limb- 

 buds the ectoderm consists of several layers of epithelial cells (Fig. 19). 

 2 



