zw 



650 EMBRYOLOGY : CHICK 



mesoderm bounded by a dotted line in Fig. 503, 3 and 4) and the 

 notochordal cells form the primitive knot, which gradually moves 



backwards, so that the noto- 

 chord is left in position and the 

 streak is obliterated. The noto- 

 chordwhen fully formed extends 

 both fore and aft a little beyond 

 the limits of the original primi- 

 tive streak. A split which forms 

 in the mesoderm sheets gives 

 rise to the coelom (Fig. 511, A,) 

 with splanchnic layer of meso- 

 derm within (below) it and 

 somatic layer without (above). 

 As in the frog (p. 634), each 

 mesoderm sheet gives rise along 

 its median side to a row of 

 epimeres ('mesoblastic somites') 

 visible chiefly as myotomes, 

 sclerotomes being indistinct, 

 and laterally to an unsegmented 

 lateral plate. By the end of the 

 first day of incubation there are 

 four or five pairs of epimeres. 



FOLDING OFF 



From the germ layers thus 

 laid down the organs of the 

 chick arise essentially in the 

 same way as those of the frog, 

 but the course of development 

 is much modified in detail, and 

 has new features added to it. 

 The protected and well- 

 provisioned egg of a bird (or 

 reptile) has great advantages in 



Fig. 508. — The origin of amnion and 

 allantois. — After Balfour. 



1. Rise of amniotic folds [a. fid.) around embryo; 



p.p., pleuro-peritoneal cavity or ccelom ; y., 

 yolk. 



2. Further growth of amniotic folds [a.fld.) over 



embryo and around yolk, 



3. Fusion of amniotic folds above embryo ; a.p., 



amnion proper ; s.s.m., false amnion or serous 

 membrane ; y.s., yolk sac. 



4. Outgrowth of allantois (oZ.) ; a.c, aimniotic 



cavity ; h., head end ; t., tail end. 



5. Complete enclosure and reduction of yolk sac 



(y.s.) ; s.z.tn., serous membrane ; a.p., amnion 

 proper ; cd., allantois ; g., gut of embryo. 



that it avoids the perils and 

 delay of the larval stage of an amphibian and the necessity of 

 visiting water at the breeding season ; but, as we have just 

 learnt, it entails forming the embryo from a thin blastoderm. 



