COLUMBA. 379 
are optical distinctions and there is no real morphological distinction 
between the two areas. 
Fig. 274.—CROsSs-SECTION THROUGH A BLASTODERM OF 
ABOUT 24 HOURS. 
Epiblast. 
Hypoblast. Mesoblast. 
Yolk. 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 EmpBryo CHICK 
OF THE SECOND Day. (Ad nat.) 
(Slightly diagrammatic.) 
Nerve Cord. Protovertebra. 
Notochord. Amniotic Fold. 
Lateral Fold of 
mnion, 
Extra-embryonic 
Ccelom. 
Mesoblast. 
Epiblast. 
Serosa. 
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. 
