-1^ 



FIRST CLEAVAGE 



B 



SECOND CLEAVAGE 



THIRD CLEAVAGE 



segmentation cavity yolk 



EARLY CLEAVAGE 



entoderm cells which will move into hypoblast by ingression 



EARLY BLASTODISC 



spread of blastodisc 



forwa 



BLASTULA 



mesent( 



concrescence toward primitive streak 



G BEGINNING OF GASTRULATION 



. J Hensen's node primitive pit 

 notochord / , primitive groove (blastopore) 



ectoderm 



mesoderm -s^&'fe^ 

 entoderm 



H LONGI SECTION OF PRIMITIVE STREAK 



I CROSS SECTION THROUGH PRIMITIVE STREAK 



first intersomite fissure 



area opaco 





neural groove 



neural fold 



head fold 



proamnion 



(mesoderm free) 



vitelline vein 



vitelline artery ■'■•,'■ 



head fold 



' tail fold 







heart 



^eye • 



^9 



•^,V.;S ' ' subcephalic poc 

 area pellucida 



ket ■■■/■■./;■■>■■;•-''' ■■■<.'-~:.'/''ii'/"''- ^•^.■•■••''.■■■■•■i"'.i-'j--'~"-'.'--:'--.';''i^-'' .•''"■• '"'■•■■ 



primitive streak (and groove) Hensen's node 



J SURFACE VIEW OF PRIMITIVE STREAK STAGE 



terminal sinus (venous) in edge of blastodisc 

 K EMBRYO WITH HEAD AND TAIL FOLDS 



Figure 7-9. Stages of the chick. A, first cleavage groove; B, second cleavage; C, third cleavage 

 grooves; D, early blastodisc; E, blastodisc with underlying segmentation cavity; F, blostula; 

 G, beginning of gastrulation; H, diagrammatic presentation of events along a sagittal section in the 

 region of gostrulotion; I, cross section through the primitive groove; J, surface appearance of eorly 

 embryo (head to right); K, early embryo with head and tail in pockets of the germinal disc. (After 

 Huettner, 1949, and Patten, 1958) 



202 



EMBRYOGENESIS OF THE CHORDATES 



