itvV ''>c,"i^- , -'?.■• -Vt-. EPIDERMAL'fOLD^IflV 



SBE GINNING OF HE 40 FOLD '„■,;,* *;■" PROAMNION / "■'■.. •".V nSB \ 



PR MITIVE STREAK 



Fig. 232. Early post-gastrular development in the chick. (A-H represent a late 

 head-process stage — stage 5 of Hamburger and Hamilton, '51. Compare with figure 

 203D. I-L show the beginnings of the head fold — intermediate condition between stages 

 7 and 8 of Hamburger and Hamilton, '51.) (A) Surface view, showing primitive streak, 

 neural plate, and epidermal areas. (B-F) Cross sections of A at levels indicated on 

 G. (G) Median sagittal section of (A). (H) Same, showing presumptive, organ- 

 forming areas of entoderm notochord, pre-chordal plate, neural plate, and primitive-streak 

 mesoderm. (I) Surface view, demonstrating a marked antero-posterior extension of the 

 neural plate area and beginnings of neural folds. Observe shortening of primitive streak. 

 (J) Drawing of stained specimen. (K) Median sagittal section of (J). (L) Same, 

 showing major organ-forming areas. In (G) and (H) the entoderm, notochord, and 

 overlying neural ectoderm are drawn as separate layers. Actually, however, at this stage, 

 the three layers are intimately associated. 



480 



