THE BLOOD. 75 



For the purpose of showing it satisfactorily the egg should 

 be broken at the side, and a portion of the shell cautiously 

 removed, without at the same time raising with it the subjacent 

 membrane (membrana testae) ; this should next be peeled off 

 with the same degree of caution as that with which the shell 

 itself was previously raised. 



Immediately beneath this membrane the yolk itself will be 

 seen floating ^in the midst of the colourless albumen, and 

 sustained in position by the beautifully spiral chalazce, which 

 proceeding from the yolk are fastened into that portion of 

 the membrana testce which corresponds with the poles of the 

 egg-shell. * 



Imbedded in the surface of the yolk of an egg on the 

 third, fourth, and fifth days of its incubation, the embryo 

 will be visible, and issuing from its umbilicus will be seen the 

 vessels which ramify in such graceful order through the 

 membrane of the allantois. 



The embryo is almost invariably placed uppermost in the 

 yolk, so that it most generally presents itself beneath, what- 

 ever part of the shell has been broken. This position results 

 from the lighter specific gravity, and is, moreover, facilitated 

 by the spiral formation of the chalazae. 



The purposes fulfilled by this position of the embryo are 

 obvious and striking, it being thus so placed as to receive 

 directly the caloric which is continually emanating from the 

 parent hen, and being also more immediately submitted to 

 the influence of the oxygen of the air. 



In an embryo then thus placed in situ, in the third, fourth, 

 and fifth days of its development, and with the unaided sight, 

 the rudiments of almost all the organs and members may be 

 clearly recognised, the eye and the regular contractions of the 

 heart, together with the vessels departing from it to ramify 

 through the allantois, being particularly conspicuous. With 

 a low power of the microscope the course of the blood in 

 the vessels, together with the form and size of the white 

 and red corpuscles, may be clearly distinguished. 



The ramifications of the vessels in the allantois presents an 

 arborescent distribution ; their entire course may be traced 



