TUBULATION OF ORGAN-FORMING AREAS 



491 



EXTR A- E MBRYON 



ENTODERMAL TUBULATION 

 DORSAL ARCHING MOVEMENT 

 DERMATOME 

 MYOTOME 

 SCLEROTOME _ 

 NEPHROTOME 



MESODERMAL TUBULATION 

 DORSAL EVAGINATION OF ENTODERM 



ATERAL FOLDS OF AMNION 



EMBRYONIC COELOM 



EXTRA-EMBRYONIC COELOM 



FUSION OF BODY LAYERS 



DISAPPEARANCE OF VENTRAL MESENTERY 

 OR VENTRAL FUSION OF MESODERMAL 

 LAYERS BELOW GUT TUBE 



Fig. 241. Formation of ventral body wail, differentiation of somites, formation of 

 dorsal and ventral mesenteries, embryonic and extra-embryonic coelom, etc., in chick 

 embryo. (A) Dorsal upgrowth is evident as neural tube, somites, and forming ento- 

 dermal (gut) tube are projected upward above the level of the extra-embryonic tissues. 

 Observe heavy line at left, denoting general region of demarcation between embryonic 

 and extra-embryonic tissues. (B) Separation of differentiating somite from nephro- 

 tome; sclerotomic mesenchyme is migrating from somite to notochordal-neural area; 

 lateral body folds are migrating mediad to form ventral wall of trunk region; lateral 

 folds of amnion are migrating dorsad. (C-E) Dorsal upgrowth movement lifts em- 

 bryonic body above extra-embryonic tissues below; fusion of ventral, body-wall layers 

 begins. (C) Body layers are meeting in midventral line. (D, E) Fusion of ventral 

 body-wall layers, disappearance of ventral mesentery. 



epimeric mesoderm on either side of the notochord gradually moves laterally 

 and dorsally and comes to lie along the lateral aspects of the notochord and 

 neural tube. During this migration, each epimere increases in thickness and 

 becomes segmented into small oblong blocks of cells called somites (figs. 23 IN; 

 233B; 234D; 245A). A somite which forms in the epimere on one side of 

 the notochord always has a corresponding somite in the epimere on the other 

 side of the notochord. Somites thus form in pairs, each pair representing a 

 primitive segment of the developing body. This primitive segmentation is a 

 fundamental characteristic of the vertebrate body. It begins in the general 



