THE EARLY DEVELOPMENT OF THE CHICK 269 



clearly mesodermal. As the primitive streak shortens and the 

 primitive knot moves posteriorly, the notochord continues to 

 differentiate, and so to elongate posteriorly. 



The cavity of the fore-gut is the rudiment of the pharynx. 

 It remains a wide but shallow cavity throughout these early 

 stages, open posteriorly, by way of the anterior intestinal portal, 

 upon the surface of the yolk, where the endoderm remains 

 spread out as a thin flat sheet (Figs. 97, 98, 103). Near the 

 anterior extremity of the head-fold, the endoderm lining the 

 pharynx comes ventrally into contact with the ectoderm, and 

 later fuses with it. This forms the oral plate, which becomes the 

 inner wall of the stomodceum; it is perforated early the third 

 day of incubation. There is a pair of lateral extensions of the 

 fore-gut, where its walls come into contact with the ectoderm, 

 marking the positions of the future first gill pouches. 



4. Nervous System 



The rudiments of the central nervous system are the most 

 conspicuous structures of the chick embryo during its early 

 development. We have already noted the formation of the 

 medullary plate, a broad ectodermal thickening developing 

 precociously, before any other part of the embryo is definitely 

 established (Figs. 94, 101). The medullary plate is converted 

 into the medullary tube in the usual manner. A medullary 

 groove appears soon after the head-fold becomes marked; the 

 marginal medullary folds then become elevated and grow 

 rapidly so that they form a pair of high conspicuous ridges 

 on the surface of the anterior part of the blastoderm. They 

 extend posteriorly, gradually diminishing in height, until 

 finally they sink into the general level of the blastoderm in the 

 region of the primitive knot. The formation of the medullary 

 folds and central nervous system in general, as of the somites, 

 notochord, and other parts of the embryo, is progressive pos- 

 teriorly, so that steps in the formation of all these parts can be 

 read in any series of transverse sections extending from the 

 primitive knot anteriorly. 



