5^6 EMBRYOLOGY I CHICK 



mesoderm with the latter. In the end it completely Hnes the shell. 

 It becomes very vascular, and by its means the embryo breathes 

 through the porous shell, the urine is passed into it and uric acid 

 deposited and retained in it, and it absorbs for the embryo water 

 and protein from the white. 



ORGANOGENY : NERVOUS SYSTEM 



The formation of the organs of the chick resembles in broad 

 outline that of the frog (compare Fig. 498 and Fig. 503, 5). 

 A neural (medullary) plate (Fig. 503, 3, n.p.) appears in the ecto- 

 derm in front of the primitive streak : its sides grow as neural 

 folds (Figs. 509, 511, n.f.) which presently, as in the frog, meet 

 above the back to form the central nervous system. The neural 

 folds lengthen backwards, and their lengthening keeps pace with 

 the transformation of the primitive streak just mentioned, so 

 that the streak is always behind the folds. By the end of the first 

 day the medullary folds have met in the region of the hind-brain. 

 On the second day the front of the head begins to bend down, 

 setting up a cranial flexure Hke that of the frog, by which the fore- 

 brain is bent down at right angles to the parts behind it (Fig. 

 512, /.6.). The further development of the nervous system takes 

 place in practically the same manner as in the frog (pp. 635, 639). 



ALIMENTARY CANAL 



The alimentary canal, hke that of the frog, arises in three 

 sections, {a) The greater part is hned by endoderm and is known 

 as the mesenteron. It arises by a folding of the splanchnopleure 

 in a manner which has been described above (p. 651). Five 

 visceral clefts, which do not bear gills and of which the 

 hinder two pairs do not break through, arise in its anterior 

 portion (pharynx), (h) A small section which forms part of the 

 mouth is lined by ectoderm and known as the stomodaeum. It 

 arises by outgrowths which form the jaws, surrounding a shallow 

 depression. A hollow diverticulum from its roof forms the hypo- 

 physis, (c) Another small section at the hind end, which forms the 

 proctodseum, is also hned by ectoderm. The stomodaeum breaks 

 through to the mesenteron at the end of the third day, but the 

 proctodseum not until the fifteenth day. 



