332 ANATOMY OY THE DOMESTIC FOWL 



the 50 per cent, alcohol and leave twelve hours. Then wash in 50 

 per cent, alcohol; after which transfer to 70 per cent, alcohol for six 

 hours. Clarify in oil of cedar or oil of cloves, and place in melted 

 paraflSn for two hours. Imbed in paraffin, and section. 



Embryos may be hardened, imbedded, and sectioned after the 

 usual methods, using either paraffin or celloidin, and the sections 

 stained with hematoxylin and eosin after sectioning. 



POINTS TO BE OBSERVED IN THE STUDY 



I. The embkyo twenty-nine to TmRTY-FOim hours old with 



FROM TEN TO FOURTEEN SOMITES. 



A study of the egg. 



In opening the egg, observe that the shell membranes are double. 



Observe that the shell is porous. 



Note the air cell at the large end of the egg and note that the space 

 or cell lies between the outer and the inner shell membrane. 



After the egg contents have dropped into the salt solution, note 

 extending from the ends of the yolk the twisted denser cords of 

 albumen. These are the chalazse, which act as stays to the yolk. 

 Note that the yolk is surrounded by a dehcate membrane. This 

 is the vitelline menibrane. The yolk is the true ovum and serves 

 as food for the developing embryo. 



A study of the Hving embryo. 



Note the amount of yolk that is covered by the blastoderm. 

 Note the shpper-shaped, transparent center of the blastoderm. 

 This is the area pellucida. In" the center of this there is a narrow 

 white streak. The area, opaca is the area lying external to the area 

 pellucida. In this there is the area vasculosa. 



A study of the embryo entire, including the vascular area. 



The blood islands show as irregular deeply stained masses in the 

 vascular area. At this stage they are inclosed in wide anastomosing 

 tubes, the extra-embryonic blood-vessels, which open peripherally 

 into the boimding sinus termiaalis. 



The following structures may be identified: 



The neural tube forming the axis of the embryo. In the anterior 

 region may be noted the forebrain, the optic vesicles, the midbrain. 

 The hindbrain is subdivided into the neuromeres. The cord, or 

 myelon, of the neural tube back of the hindbrain is closed in front 

 but is open behind. 



