152 



DEVELOPMENT DURING THE FOURTH DAY. 



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 segmental tubes (fig. 116 st] and connective tissue 

 of the Wolffian body. 



Various other features in the development of the vascular system, 

 general mesoblast, etc., are also represented in these sections. It may 

 more especially be noted that there are at first two widely separated 

 dorsal aortoe, which gradually approach (fig. 115 and 116); and meeting 

 first of all in front finally coalesce (fig. 117 & 119) for their whole length. 



The general appearance of the embryo of the fourth day may be 

 gathered from fig. 118. 



The changes which have taken place consist for the most part in 

 the further development of the parts already present, and do not 

 need to be specified in detail. The most important event of the day is 



nc. 



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



Or THE FOURTH DAY. 



n,c. neural canal ; p.r. posterior root of spinal nerve with ganglion ; ..?. 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; in. p. muscle-plate; cli. uotochord ; TIM?. Wolffian ridge. 

 A.O. dorsal aorta ; V.c.a. posterior cardinal vein; W.d. Wolffian duct; W.li. Wolffian 

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

 mentary canal; M. commencing mesentery; SO. somatopleure ; SP. splanchno- 

 pleurc ; I', blood-vessels; pp. pleuroperitoneal cavity. 



