THE MUSCLE-PLATES. 



135 



SO 



SECTION THROUGH THE DORSAL KEGION OP AN EMBRYO AT THE END OF THE 



THIRD DAY. 



Am. amnion. m. p. muscle-plate. C. F. cardinal vein. Ao. dorsal aorta. The 

 section passes through the point which the dorsal aorta is just commencing 

 to divide into two branches. Ck. notochord. W. d. Wolffian duct. W. b. 

 commencing differentiation of the mesoblast cells to form the Wolffian 

 body. ep. epiblast. SO. somatopleure. Sp. splanchnopleure. hy. hypoblast. 

 The section passes through the point where the digestive canal communicates 

 with the yolk-sac, and is consequently still open below. 



This section should be compared with the section through the dorsal region of 

 an embryo at the commencement of the third day (Fig. 41). The chief differences 

 between them arise from the great increase in the space (now filled with meso- 

 blast-cells) between the notochord and the hypoblast. In addition to this we 

 have in the later section the completely formed amnion, the separation of the 

 muscle-plate from the protovertebrse, the formation of the Wolffian body, etc. 



The mesoblast including the Wolffian body and the muscle-plate (m.p.) is 

 represented in a purely diagrammatic manner. The amnion, of which only the 

 inner limb or true amnion is represented in the figure, is seen to be composed of 

 epiblast and a layer of mesoblast ; though in contact with the body above the top 

 of the medullary canal, it does not in any way coalesce with it, as might be con- 

 cluded from the figure. 



