THE EARLY DEVELOPMENT OF THE CHICK 261 



and is an extension of the archenteric space between endoderm 

 and yolk; the other occupies the ventral limb, is lined with ecto- 

 derm, and is continuous with the space entirely outside the 

 embryo. The dorsal, endodermally lined cavity is the rudiment 

 of the fore-gut. The lower space is sometimes called the cavity 

 of the head-fold; it is merely an external space lying under the 

 head of the embryo. The boundary between embryonic and 







FIG. 98. Median sagittal section through the head end of an embryo with 

 four pairs of somites (twenty-three to twenty-four hours). From Lillie (Devel- 

 opment of the Chick), a.i.p., Anterior intestinal portal; Ect. ectoderm; Ent., 

 endoderm; F.G., fore-gut; H.F., head-fold; med. pi., anterior limit of medullary 

 plate; Mes., mesoderm; Mes. H.G., mesodermal head cavity; n. F., neural fold: 

 or. pi., oral plate (oral membrane). 



extra-embryonic structures is just along the middle, i.e., most 

 posterior limit, of this latter fold or cavity; at this level the 

 morphologically anterior limit of the medullary plate passes 

 abruptly into the extra-embryonic ectoderm of the blastoderm, 

 in the region of the proamnion. 



Laterally the head-fold and fore-gut narrow and finally dis- 

 appear into the flat blastoderm. The posterior curvatures of 

 the lateral extremities of the head-fold mark the lateral sur- 



