76 ORGANISED FLUIDS. 



from their commencement in the aorta to their termination 

 on the border of the membrane of the allantois. 



Now the great point of interest in the circulation of the 

 chick is that the passage of the blood may be witnessed 

 throughout ; thus the blood expelled from the heart by the 

 contraction of the ventricle into the aorta may be traced 

 through this vessel, and all its subsequent divisions and sub- 

 divisions, until it reaches the ultimate arterial radicles, passes 

 from these into the corresponding radicles of the veins, and 

 from these again into the larger venous trunks, by which it 

 is reconveyed to the heart, the circle of the circulation being 

 thereby completed. 



There are two ways in which the circulation in the em- 

 bryo of the fowl may be viewed, either while it is still 

 occupying its natural position on the surface of the yolk, 

 and this I think is by far the most preferable method ; or the 

 embryo may be altogether detached from the yolk by means 

 of an armed needle, and subsequently placed on a watch-glass 

 filled with warm water at a temperature of 96. During the 

 operation of detaching the embryo, the egg itself should also 

 be immersed in water at the temperature just mentioned. 

 This latter process is one, however, of much nicety, and 

 frequently fails in consequence of the rupture of some of the 

 finer vessels, the blood becoming effused, the different parts 

 of the embryo obscured, and a stoppage put to the cir- 

 culation. 



But it is not alone the contemplation of the circulation in 

 the embryo of the chick which is so interesting and in- 

 structive ; the study of the entire development of the ovum 

 from its commencement to its termination reveals facts of 

 the highest importance and full of wonder. 



The examination of the blood of the embryo fowl is 

 especially instructive, the mode of formation of the red cor- 

 puscles admitting of determination in a manner the most 

 satisfactory. 



In the red corpuscles contained in the blood of the embryo 

 in the first days of its development, a remarkable variation of 

 size will be detected, some of them being three or four times 



