Stages in the development of a chick. — After Marshall. 



Segmentation, superficial view of blastoderm. 2. Vertical section of blastoderm. 

 Ep., Epiblast ; I.e., lower layer of cells ; s.g.c, sub-germinal cavity ; y., yolk. 

 3. Diagrammatic surface view; a.p., area pellucida ; a.o. y area opaca ; «./., 

 neural groove ; fi.s., primitive streak ; M., mesoblast spreading over yolk. 4. 

 Diagrammatic surface view of later stage; a.p., area pellucida; a.o., area 

 opaca; m.s., mesoblast segments; p.s., primitive streak. Tbe dark border 

 shows the spreading of the mesoblast over the yolk. 5. Cross-section ; .i.e., 

 spinal cord ; s.g-., rudiment of spinal ganglia ; N., notochord ; m.ft., muscle plates; 

 A., aorta ; Am., amnion fold ; c. , ccelom or pi euro -peritoneal cavity. 6. Embryo, 

 CIk, Cerebellum ; F., ear ; H., heart ; f.l. , fore-limb ; h.I., hind-limb ; y.s., stalk 

 of cut-off yolk sac; Al., allantois ; E., eye; C, cerebrum. On the dorsal 

 surface the mesoblastic somites are indicated. 



33 



» 



