490 



DEVELOPMENT OF PRIMITIVE BODY FORM 



4. TUBULATION (COELOM FORMATION) AND OtHER FEATURES 



Involved in the Early Differentiation of the 

 Mesodermal Areas 



The differentiation of the mesodermal areas is an all-important feature of 

 embryonic development, for the mesoderm contributes much to the substance 

 of the developing body. (See Chaps. 11 and 15.) While the neural, enteric, 

 and epidermal tubes are being established, radical changes occur within the 

 two mesodermal layers on either side of the notochord as follows: 



a. Early Changes in the Mesodermal Areas 



1) Epimere; Formation of the Somites. The longitudinal mass of paraxial 

 mesoderm which lies along the side of the notochord forms the epimere 

 (figs. 221F, G; 234E, F). The two epimeres, one on either side of the noto- 

 chord, represent the future somitic mesoderm of the trunk area. In the early 

 post-gastrula, the epimeric mesoderm, together with the notochord, lies im- 

 mediately below the neural plate. However, as neuralization is effected* the 



SCERAL) ARCH 



OlO 

 CONO VISCERAL) 

 ARCH 



MANDIBULAR 



PROCESS 

 MAX I LL ARY 



PROCESS 

 MANOIBUL AR 

 R FIRST 

 SCERAL ARCH 



STERIOR LIMB BUD 



Fig. 240. Chick embryo of about 72 to 75 hours of incubation, about stage 20 of 

 Hamburger and Hamilton, '51. 



