API'.] THIRD DAY EMBRYO. 447 



2. Through auditory sac, shewing : 



a. Auditory sac still open. 



h. The thin roof and thick sides of the hind-brain. 



c. Notochord. 



d. Heart. 



e. Closed alimentary canal. 



3. Through dorsal region, shewing the general appear- 

 ance of a section of an embryo at this stage, which 

 should be compared with a similar section of the 

 earlier stage. 



It shews : 



a. The commencement of the side folds; the ali- 

 mentary canal still however open below. 



h. The Wolffian duct lying close under the epiblast 

 on the outside of the mesoblastic somites. 



c. The notochord with the aortse on each side. 



IV. Examination of an Embryo at the end of the third 



day. 



A. Opening the egg, as in II. A. 



B. Examination of the blastoderm in situ. 

 Observe : 



1. The great increase of the vascular area both in size 

 and distinctness. The circulation is now better 

 seen in situ than after the blastoderm has been 

 removed. 



2. That the embryo now lies completely on its left 

 side and that it is only connected with the yolk-sac 

 by a somewhat broad stalk. 



