THE MESODEEM. 



29 



region of the head ; the remainder are in the body area of the embryonic region. 

 The segmentation of the paraxial bars commences before their elongation is com- 

 pleted, and the posterior somites are separated off as the paraxial bars are extended 

 by the continued proliferation from the nodal point at the anterior end of the 

 primitive streak. 



When they are first defined the somites are solid masses of cells, but in a short 

 time a cavity the co3lom of the somite or myoccele is developed in each mass. 



Mesoderm of amnion ~~~~- 



Ectoderm of amnion * 



Neural crest 



Roof-plate 



Lateral wall of neural 

 groove 



Floor-plate 



Mesoderm of 

 entoderm vesicle 

 Entoderm 



Cavity of entoderm 

 vesicle 



Amnion cavity 



Notochord 



Mesoderm of 



chorion 

 Trophoblast of 

 chorion 



FIG. 41. 



A. Transverse section of a zygote, showing the constituent parts. 

 B. Diagram of embryonic area showing parts of neural plate and primitive streak. 



The apical portion of the hollow mesodermal somite is its scleratogenous segment. 

 The cells of the scleratogenous section of the somite undergo rapid proliferation. 

 Some of the newly formed scleratogenous cells invade the myocoele ; others migrate 

 towards the notochord ; finally, the scleratogenous cells separate from the remainder 

 of i the somite, and as they increase in number they migrate along the sides of 



Trophoblast of chorion 

 Mesoderm of chorion 



Mesoderm of amnion 

 Ectoderm of amnion 

 Neural crest 



Roof-plate 



Lateral wall of 



neural groove 



Floor-plate 



Primitive Entoderm 



streak Mesoderm of 



entoderm vesicle 

 Cavity of entoderm 

 vesicle 



Amnion cavity 

 ^Paraxial 

 mesoderm 



Notochord 



FIG. 42. 



A. Diagram of a transverse section of a zygote, showing the formation of a neural groove in the embryonic area. 

 B. Diagram of a surface view of the embryonic area of the same zygote. 



the notochord and the neural tube, which has been formed in the meantime from the 

 neural groove, and join with their fellows of the opposite side, and with their 

 cephalic and caudal neighbours. In this way is formed, around the neural tube 

 and the notochord, a continuous sheath of mesoderm, the membranous vertebral 

 column, from which are differentiated, in later stages, the vertebral column and its 

 ligaments, and the membranes of the brain and the spinal medulla. 



