512 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



derm, float it on a slide, cover with coverslip supported by wax feet 

 at the corners and examine as a transparent object, comparing the 

 various features with those shown in Figs. 235 and 236. 



D. Excise a second embryo with its surrounding blastoderm. 

 Float it on to a coverslip, embryo above, and submerge it in a watch- 

 glass of fixing fluid. Watch it carefully under the lens or preferably 

 low-power binocular as the tissues gradually become opaque. The 

 amnion will be seen particularly clearly during this process. A 

 drawing should be made of the embryo enclosed in its amnion as an 

 opaque object. 



E. Carefully strip off the amnion with a pair of needles : and 

 study the configuration of the head end of the embryo. 



F. Stain and mount the embryo. 



G. Prepare series of sections (a) transverse to trunk region, (b) 

 horizontal through trunk region and therefore approximately sagittal 

 in the region of the head which is lying over on its left side. 



VI. The Foukth Day. — On placing the egg in the salt solution 

 the broad end will tilt up more decidedly than before owing to the 

 increase in size of the air space. Care should therefore be taken to 

 make the first perforation of the shell close to the broad end so as to 

 allow the air to escape. Care must also be taken not to injure the 

 vascular area as the whole blastoderm is now much closer to the 

 shell than it was in earlier stages. As soon as the egg has been 

 opened a careful drawing should be made while the embryo is still 

 alive and in situ. The main features of the vascular system in par- 

 ticular should be carefully worked out at this stage. If the circula- 

 tion becomes sluggish through cooling a little warm salt solution 

 should be added but care must be taken not to bring about a great 

 and sudden rise of temperature as in this case the greatly accelerated 

 heart-beat is apt to cause rupture of a vessel. 



The body of the embryo, allantois, etc., are covered over by the 

 thin transparent serous membrane or false amnion as becomes 

 apparent if the attempt is made to push a blunt needle down into 

 the space round the allantois. This membrane should either be cut 

 through with a pair of fine scissors, just outside the boundary of the 

 allantois, or carefully stripped off with fine forceps. When this has 

 been done it is possible to shift the body of the embryo into such a 

 position that it with its blood-vessels can be observed in side view. 

 Until this has been done it is impossible to get a proper view of the 

 body of a well-developed embryo of this age owing to its dipping 

 down out of sight into the yolk-sac. 



The embryo should now be excised by cutting round outside the 

 boundary of the vascular area and floated into a watch-glass of clean 

 warm salt solution. The embryo may now be studied as a trans- 

 parent object on the stage of the dissecting microscope. It is better 



1 Bearing in mind that steel needles must not be allowed to touch corrosive sub- 

 limate solutions. Picric acid solutions are convenient fixing agents to use for D 

 and E. 



