AVES. 



183 



rentiation of the mesoblastic somites (fig. 115, P.v) into (a) the 

 muscle-plates (figs. 116, ms and 117, m.p), and (b) the tissue to 

 form the vertebral bodies and adjacent connective tissue ; (2) the 

 formation of a mass of tissue between the lateral plates and the 

 mesoblastic somites (fig. 115), known as the intermediate cell 

 mass, on the dorsal side of which the Wolffian duct is formed, 

 while the intermediate cell mass itself breaks up into the seg- 

 mental tubes (fig. 116, si) and connective tissue of the Wolffian 

 body. 



W.P 



FIG. 119. SECTION THROUGH THE LUMBAR REGION OF AN EMBRYO CHICK AT THE 



END OF THE FOURTH DAY. 



n.c. neural canal; p.r. posterior root of spinal nerve with ganglion; a.r. anterior 

 root of spinal nerve; A.G.C. anterior grey column of spinal cord; A.W.C. anterior 

 white column of spinal cord just commencing to be formed, and not very distinctly 

 marked in the figure; m.p. muscle-plate; ch. notochord; W.R. Wolffian ridge; 

 AO. dorsal aorta; V.c.a. posterior cardinal vein; W.d. Wolffian duct; W.b. Wolffian 

 body, consisting of tubules and Malpighian bodies; g.e. germinal epithelium; d. ali- 

 mentary canal ; M. commencing mesentery; SO. somatopleure ; SP. splanchnopleure; 

 V. blood-vessels ; //. pleuroperitoneal cavity. 



