36 



HUMAN ANATOMY. 



a very early stage of development it is a reasonable inference that in man such 

 a process actually occurs. 



In diagram B the niesoblast has not only surrounded the entoblastic sac and 

 the inner surface of the trophoblast, so enclosing the large extra-embryonic ccelom, 

 but has invaded the layer of cells above the amniotic cavity, dividing this layer into 

 two parts, the inner part going to form the ectoblast of the amnion, the outer part 

 being a continuation of the trophoblast of the chorion There is here evidently a 

 very great development of the extra-embryonic coelom. In explanation of this 

 condition, it may be assumed that the entoblastic sac is at first much smaller than the 

 trophoblastic covering of the vesicle ; that the mesoblast, shortly after its appearance. 



Fig. 43. 



Diagrams illustrating development of human fata! membranes. Stage A is hypothetical ; others are based on 

 stages which have been actually observed. Red represents trophoblast; purple, embryonic ectoblast; gray, meso- 

 blast ; blue, entoblast. ac, amniotic cavity ; a/, allantois ; am, amnion ; b, body-stalk ; ch, chorion ; ee, embryonic 

 ectoblast; en, entoblast; g, gut-tube; w, mesoblast ; />, placental area; <, trophoblast ; !», yolk-sac ; z/6^, yolk-stalk. 



develops a coelom ; that the two layers of the mesoblast so formed grow separately 

 around the vesicle ; the splanchnic layer around the entoblast, the somatic layer 

 around the trophoblast, so enclosing between them as they grow, the considerable 

 space which becomes, by this process, extra-embryonic body cavity. This diagram 

 corresponds roughly to the condition of Peters' embryo (Fig. 44). The trophoblast 

 is greatly thickened ; its outer surface very irregular, showing lacunae or spaces 

 filled with maternal blood. This early intimate contact of the foetal tissue with the 

 maternal blood permits nutrition of the young embryo from the maternal blood to be 

 carried on through the trophoblast cells some time before the allantoic circulation 

 and definite placenta are established Hence the significance of this term trophoblast. 



