COLUMBA. 



379 



are optical distinctions and there is no real morphological distinction 

 between the two areas. 



Fig. 274. — Cross-section through a Blastoderm of 



ABOUT 24 HOURS. 



Neural Groove. 



Hypoblast. 



Epiblast. 



Yolk. 



Mesoblast. 

 Archenteron. Notochord. 



A shows the whole blastoderm lying on the yolk. B shows the median 

 part only more highly magnified. 



Fig. 275. — Transverse Section of an Embryo Chick 

 OF THE Second Day. {Ad nat.) 



(Slightly diagrammatic.) 



Lateral Fold of 

 Amnion. 



Ex tra - embryonic 

 Coelom. 



Nerve Cord. 

 Notochord. 



Protovertebra. 



Amniotic Fold. 



Mesoblast. \ g 

 Epiblast. j ^ 



Hypoblast of Yolk-sac. 



Splanchnic Mesoblast. 



The first appearance of the embryo is the neural tube which arises 

 immediately in front of the primitive streak. Paired neural folds grow 

 up to form a neural tube and eventually enclose the primitive streak. 



