150 SECTIONS DURING THE SECOND AND THIRD DAY. 



Part of the membranous mandibular and byoid arches form a wall round 

 the dorsal part of the original opening of this cleft, and so give rise to the 



Fig. 117- Section through the dorsal region of an Embryo Chick at the end 



or THE THIRD DAY. 



Am. amnion; m.p. 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. Ch. 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. 



meatus auditorius externus. At the bottom of this is placed the tympanic 

 membrane, which is probably derived from the tissue which grows over the 

 dorsal part of the opening of the first cleft. It is formed of an external 

 epiblastic epithelium, a middle layer of mesoblast, and an internal hypo- 

 blastic epithelium. 



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. 



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 con- 



