646 EMBRYOLOGY : CHICK 



By a further series of horizontal clefts the disc becomes two or 

 three cells deep (C). In that way, shortly before the laying of 



1. 



Fig. 503. — Stages in the development of a chick. — After Marshall. 



Segmentation ; superficial view of blastoderm. 2. Longitudinal vertical section of later blastoderm : 

 blc, Blastocoele ; ep., ectoderm ; g.w., germinal wall ; I.e., lower layer of cells ; s.g.c, sub-germinal 

 cavity ; y., yolk. 3. Diagranmiatic surface view, a little later ; a.p., area pellucida ; a.o., area opaca ; 

 n.p., neural plate ; p.s., primitive streak ; M., mesoderm spreading from primitive streak, seen through 

 ectoderm. 4. Diagrammatic surface view of later stage (end of the first day) ; a.p., area pellucida ; 

 a.o., area opaca ; m.s., mesoblastic somites ; p.s., primitive streak. The dark border shows the spreading 

 of the mesoderm over the yolk. 5. Cross-section behind the heart, at the end of the second day ; 

 A., one of the vessels which join to form the dorsal aorta ; am., amnion fold ; c, ccelom or pleuro- 

 peritoneal cavity of the embryo ; c^., extra-embryonic coelom ; ep., ectoderm ; hy., endoderm ; s.c, 

 spinal cord ; s.g., dorsal root ganglion forming from neural crest ; so., somatic mesoderm ; sop., somato- 

 pleure ; spl., splanchnic mesoderm, with blood vessels; spp., splanchnopleure ; N., notochord; 

 m.p., myotome ; W.d., Wolffian duct, with rudiment of kidney tubule adjoining. 6. Embryo at the 

 end of the fifth day ; a.v., auditory vesicle ; al., allantois ; C, cerebrum ; cb., cerebellum ; E., eye: 

 /./., fore-limb ; H., heart ; h.l., hind-limb ; o.L, optic lobe ; pn., pineal body ; y.s., remajns of stalk of 

 cut-off yolk sac ; tag., vagus. 



