Fig. 237. Chick embryo of 17 to 19 pairs of somites. (Approximating Hamburger 

 and Hamilton, '51, stage 13, 48 to 52 hours of incubation, sections indicated on outline 

 drawing.) Head lies partly on left side; auditory pits are deep; cervical flexure is evident 

 in region of rhombencephalon; cephalic flexure is marked; stomodaeum is a deep inden- 

 tation touching foregut between the first pair of aortal arches; head fold of amnion 

 reaches back to anterior part of rhombencephalon (hindbrain). (A) Anterior (telen- 

 cephalic) portion of prosencephalon, showing closed neuropore; amnion is indicated. 

 (B) Optic vesicles. (C) Anterior end of foregut, showing anterior extremity of sto- 

 modaeal invagination and first (mandibular) pair of aortal arches; notochord ends and 

 pre-chordal plate area begins at about this section. (D) Anterior end of heart (ventral 

 aorta); observe thin roof plate of neural tube, characteristic of the later myelencephalic 

 (medulla) portion of rhombencephalon or hindbrain. (E) Otic (auditory) pits and 

 anterior region of ventricular portion of heart. (F) Caudal limits of forming heart, 

 dorsal mesocardium, neural crest cells. (G) Caudal end of heart, showing converging 

 (vitelline) veins of the heart, sclerotome given off to notochordal area, lateral meso- 

 cardium forming. (H) Anterior trunk area, showing diff'erentiation of somite and 

 typically flattened condition of ectoderm, mesoderm, and entoderm. (I) Caudal trunk 

 area, showing undifferentiated somite (epimeric mesoderm), intermediate mesoderm 

 (mesomere), and lateral plate mesoderm (hypomere). (J) Similar to (I). (K) Caudal 

 trunk region, showing closing neural tube. (L) Area of Hensen's node. (M) Primi- 

 tive streak. 



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