28 



HUMAN EMBEYOLOGY. 



with one another round the medial border of the coelom. The lateral border of the 

 somatic mesoderm is continuous, at the margin of the embryonic area, with the 

 mesoderm which covers the outer surface of the amnion, and the lateral border 

 of the splanchnic layer is continuous with the mesoderm on the wall of the 

 extra-embryonic or yolk-sac portion of the entodermal sac. 



The Paraxial Mesoderm. Each paraxial mesodermal bar soon assumes the form 



Trophoblast cellular layer 

 Plasmodial trophoblast x 

 Neural tube 



Mesoderm of chorion 



Mesodermal somite 

 Notochord 



Intermediate cell tract 



Amnion cavity 



Amnion 



Somatic mesoderm 



Coelom 



Splanchnic mesoderm 



Primitive gut 



Extra-embryonic coelom' 

 Wall of yolk-sac 



Cavity of yolk-sac 



FIG. 39. TRANSVERSE SECTION OF THE ZYGOTE SHOWN IN FIG. 38, showing the differentiation 



of the mesoderm. 



of a triangular prism with the apex directed ventro- medially, towards the notochord, 

 and the base dorso-laterally, towards the surface ectoderm. 



The cephalic portion of each paraxial bar, as far caudalwards as the middle 

 of the hind-brain, remains unsegmented, but the remainder is cut into a number of 



Chorion 



Scleratogenous mesoderm 

 Muscle plates 



Cutis lamella 

 Wolfflan duct " 

 Intermediate cell tract 



Amnion 



Amnion cavity 

 Neural tube 



Spinal ganglion 

 _ Sympathetic ganglion 



Aorta 



Intra-embryonic coelorn 

 _ Extra-embryonic coelom 



.. Gut 

 Coelom 



Umbilicus 

 .. Yolk-sac 



FIG. 40. SCHEMA OP A TRANSVERSE SECTION OF A ZYGOTE, showing differentiation of mesoderm and 



extension of amnion. 



segments, the mesodermal somites, by a series of transverse clefts (Fig. 38). The 



irst cleft appears in the region of the hind-brain, and the others are formed 



sively, each caudal to its predecessor. Only three or four somites lie in the 



