APP.] TWENTY HOURS EMBRYO. 453 



F. Sections. Manipulation as in I. B. 3. 

 The most important sections are, 



1. Through the eyes. 



2. Transverse section immediately behind the visceral 

 arches, shewing the origin of the lungs. 



3. Transverse section just in front of the umbilical 

 stalk, shewing the origin of the liver. 



4. Transverse section at about the ceutre of the 

 dorsal region, to shew the general features of the 

 fourth day. Compare Fig. 68. 



Amongst the points to be noticed in this section, are 



a. Muscle-plates. 



b. Spinal nerves and ganglia. 



c. Wolffian duct and bodies. 



d. Mailer's duct. 



e. Mesentery. 



f. Commencing changes in the spinal cord. 



5. Section passing through the opening of the allan- 

 tois into the alimentary canal. 



For the points to be observed in embryos of 

 the fifth and sixth, days, the student must consult 

 the chapters devoted to those days. 



In the hardened specimens, especial attention 

 should be paid to the changes which take place in 

 the parts forming the boundaries of the mouth. 



VT. Examination of a Blastoderm of 20 hours. 



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



B. Examination in situ. 



It will not be found possible to make out anything 

 very satisfactory from the examination of a blasto- 



