AVES. 



181 



The general nature of the changes, which take place in the 

 trunk between the commencement of the second half of the 

 second day and the end of the third day, is illustrated by the 

 sections figs. 115, 116, 117. 



e// - 



SO 



FIG. 117. SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK AT 



THE END OF THE THIRD DAY. 



Am. amnion ; /./. muscle-plate. C. V. cardinal vein. Ao. dorsal aorta. The 

 section passes through the point where the dorsal aorta is just commencing to divide 

 into two branches. CJi. notochord ; W.d. Wolffian duct ; W. b. commencing differen- 

 tiation of the mesoblast cells to form the Wolffian body; ep. epiblast; So. somato- 

 pleure ; Sp. splanchnopleure; hy, hypoblast. The section passes through the point 

 where the digestive canal communicates with the yolk-sack, and is consequently still 

 open below. 



In the earliest of these sections there is not a trace of a fold- 

 ing off of the embryo from the yolk, and the body walls are 

 quite horizontal. In the second section (fig. 116), from an 

 embryo of about two days, the body walls are already partially 

 inclined, and the splanchnopleure is very distinctly folded 

 inwards. There is a considerable space between the notochord 

 and the hypoblast, which forms the rudiment of the mesentery. 



