INCUBATION OF AN EGG 



form a concavity in the middle part, their margins 

 coming together under an acute angle, comparable 

 to the shape of a lance head, the embryo occupy- 

 ing the middle position. The groove that is to be- 

 come the vertebral canal becomes distinguishable, and 

 the latter soon after is formed. The cicatricula is 

 now .016, the pellucid area .006 in diameter and the 

 embryo .0055 m long. Three hours later it is 

 .0065 m long. When the incubation has lasted for 

 thirty hours the commencing formation of the prin- 

 cipal organs the heart, brain, etc. can distinctly be 

 seen. At this time a vascular network commences 

 in the cicatricula. The blood divides itself to the 

 right and left of the embryo into a plexus of capil- 

 laries, that gather finally into larger vessels that carry 

 it above, or direct it below, whence it returns to the 

 heart. 



In forty-five or forty-six hours there may be seen 

 towards the abdominal region of the chick a trans- 

 parent membranous vesicle about the size of the head 

 of a pin. This develops rapidly, spreads itself over 

 the surface of the yolk, and finally invades the whole 

 inner surface of the shell, to which it attaches itself. 

 That portion of the vesicle which is in contact with 

 the shell is abundantly provided with blood vessels, 

 and it is evident that the blood which is sent thither 

 is venous, whilst that which returns is arterial. It 

 28! 



