FORMATION OF THE EMBRYO IX THE FOWI/S EGG. 631 



closely approximated, and the medullary groove, at first widely open 

 along the dorsal surface, is reduced at its opening to a comparatively 

 narrow fissure (Fig. 185). 



FIG. 185. 



Ect 



Xnt ch 



TRANSVERSE SECTION OF EMBRYO CHICK, through narrowed portion of medullary groove.^. 

 Medullary groove. Dp. Dorsal plates. Eci. Ectoderm. Md. Mesoderm. Ent. Entoderm. Ch. 

 Chorda dorsalis. p. Peritoneal space, a o. Embryonic aorta, one on each side. (Kolliker.) 



That the dorsal plates are formed, in the manner above described, 

 by folds of the ectoderm, is plain from the fact that at this time the 

 layer of ectodermic cells lining the medullary groove is reflected con- 

 tinuously on each side, at the edges of the dorsal plates, upon the adja- 

 cent free surface of the blastoderm. Finally, the dorsal plates come in 

 contact at their edges and coalesce with each other, thus obliterating 

 the fissure between them, and converting the medullary groove into a 

 closed canal (Fig. 186). When this change is accomplished, the ecto- 

 derm, which was originally continuous throughout, is divided into two 

 portions a thicker portion lining the cavity of the canal, and a thin- 



TRANSVERSE SECTION OF EMBRYO CHICK, through closed portion of medullary canal. Me. Medul- 

 lary canal. Ect. Ectoderm. Ent. Entoderm. Md. Md. Outer and inner laminae of Mesoderm. p. 

 Peritoneal space, ch. Chorda dorsalis. a o. Aorta. (Kolliker) 



ner portion covering the canal along the median line and thence ex- 

 tending laterally over the general surface of the blastoderm. The canal 

 thus formed is the. "medullary canal." The layer of cells by which 

 it is surrounded afterward produces the brain and spinal cord, or the 



