DEVELOPMENT. 



201 



The blood-vessels ramify in all directions through the 

 yolk, making it a spongy mass, and all perform the same 

 office ; it is not till the fourth or fifth day that arteries 

 can be distinguished from veins, by being thicker, and by 

 carrying blood only from the heart.'" 



A. A 



Fio. 169.— Embiyo in a HenV Egg during tlie first five days, longitiidmal view; A, 

 hyi)()bl!ist; B, lower layer of mesoblast ; C, upper layer iifmesnblast aud epiblast 

 united, in the last figures forming ilie aumititic sac; D, vileDine membrane; e, 

 thickened blastoderm, the firs^t rudiment of the dor>:al part (in the last figure it 

 mnrks the place of the luijgs); A, heart; a, 6, its two chambers; c, aortic arches ; 

 m, aorta ; i, liver ; p, allantoic. 



