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 



O^-^Q <*>O 3 >U r^O^vXOpiVvo. n~Q>r. 



H.F. 



or.fi/ 



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- 



