Development of the Third Day 215 



vided into blocks corresponding to the original 

 somites : thus in the chick embryo we have a 

 metameric arrangement of muscles similar to 

 the arrangement of the muscles in the adult 

 fish. 



The outer end of the muscle plate rapidly 



Fig. 70. — Transverse section through the dorsal 

 REGION OF AN EMBRYO OF 68 HOURS. (After Duval.) 



Am, amnion. Ao, aorta, ii and z», hypoblast, CM, neural 

 canal. CIV. Wolffian duct. CSW, nephrostome, or segmental canal 

 of Wolffian body. G/, alimentary canal. /'K, muscle plate. l^CP, 

 posterior cardinal vein. ^J, wall of umbilical stalk. 



extends into the somatopleure, or body-wall, 

 and becomes largely converted into muscle 

 fibres from which the muscles of the back and 

 trunk are formed. The median portion of the 

 mesoblastic somite, left after the formation 

 of the muscle plates, is converted, as will be 

 described later, chiefly into the vertebrae. The 



