THE MESODEKM. 



27 



Plasmodial trophoblast 



Cellular trophoblast 



- Mesoderm lining 

 trophoblast 



. 



-Chorion 



iak \ Mesoderm covering 

 lK' x-'* 



entoderm vesicle 



At the cephalic end of the embryonic area the medial margins of the 

 mesodermal sheets fuse together across the median plane, forming a transverse 

 bar of mesodermal cells which may be called the pericardial mesoderm (Fig. 48), 

 because the pericardial sac, which envelops the heart, is afterwards developed 

 from it. The area in which this mesoderm lies may be named the pericardial 

 region of the embryonic area (Fig. 48). 



Between the bar of pericardial mesoderm, the cephalic end of the neural 

 groove, and the medial margins of the mesodermal plates lies a small segment of 

 the embryonic area from which the primary mesoderm entirely disappears, leaving 

 the ectoderm and entoderm in contact. This is the bucco-pharyngeal area. It 

 afterwards becomes the bucco-pharyngeal membrane (Figs. 50, 55), which separates 

 the primitive mouth or stomatodseum from the cephalic end of the primitive 

 entodermal alimentary canal. As already stated, the bucco-pharyngeal membrane 

 disappears during the third week, when the stomatodseum and the primitive 

 alimentary canal become A 



continuous with each other. 



Between the bucco- 

 pharyngeal area and the 

 cephalic end of the primi- 

 tive streak the medial 

 margins of the mesodermal 

 plates are separated from 

 one another by the noto- 

 chord and the neural 

 groove (Fig. 36), and still 



more caudally they are ^^^R \\\ j f [ I/r~-^^^B Mesoderm of amnion 

 united with the sides of 

 the streak (Fig. 34). 



After the permanent 

 mesodermal plates are de- 

 finitely established a series 

 of clefts appear in their 

 peripheral margins. The 

 clefts, on each side, soon 

 fuse together to form the 

 bilateral rudiments of the 

 embryonic ccelom (Fig. 36). 



The septum of cells at 

 the lateral border of the 

 embryonic area on each 

 side, which, for a time, 

 separates the embryonic 

 from the extra-embryonic ccelom, soon disappears, and the ccelom then forms a 

 continuous cavity (Fig. 37). 



The embryonic ccelom also extends medially, but the medial extension ceases 

 whilst the cavity is still at some distance from the median plane, except at the 

 cephalic end of the embryonic area, where the two lateral halves of the embryonic 

 ccelom become continuous with one another through the interior of the pericardial 

 mesodermal bar (Figs. 49, 55). 



As the embryonic ccelom is forming and extending, a longitudinal constriction 

 appears in each lateral half of the mesoderm, a short distance from its medial 

 border. This constriction separates each plate into three parts : (1) a medial bar, the 

 paraxial mesoderm, which lies at the side of the neural groove and the notochord (Fig. 

 37) ; (2) the constricted portion, which is called the intermediate cell tract ; and (3) 

 the part lateral to the constriction, which is called the lateral plate (Fig. 37). 



The embryonic ccelom is confined, as a rule, in the human subject, to the lateral 

 plate, which it divides into a superficial layer, next the ectoderm, the somatic 

 mesoderm, and a deeper layer, next the entoderm, the splanchnic mesoderm. 



The medial borders of the somatic and splanchnic mesoderm are continuous 



Mesodermal somites 

 (paraxial mesoderm) 



Ectoderm of amnion 



Mesoderm of body-stalk 



FIG. 38. SCHEMA OF DORSAL ASPECT OF EMBRYO, showing partial 

 closure of neural groove. 



Portions of the choriou and amnion have been removed. 



The neural folds have fused, except in the cephalic and caudal regions, 

 both the cephalic and the caudal ends of the embryo have been bent 

 ventrically and thirteen mesodermal somites have been formed. 



