EARLY DEVELOPMENT OF THE CHICK. 81 



extent reaches from the outer edge of the nephrotome to the margin of the 

 area vasculosa. The cleft-like ccelom develops on the ectodermal side of 

 the blood islands and vessels, so that these structures are at first confined 

 to the inner layer of mesoderm which lies close to the entoderm. The 

 ccelom is the rudiment of the serous cavities of the adult the pleural, peri- 

 cardial and peritoneal cavities. The layer of mesoderm apposed to ectoderm 

 is known as the parietal or somatic layer, while that apposed to entodeim 

 is called the visceral or splanchnic layer (Fig. 52). By comparison with 

 Amphioxus and the frog it is seen that ectoderm and somatic mesoderm con- 

 stitute the body wall and that entoderm and visceral mesoderm comprise 

 the wall of the archenteron or gut cavity, although in the chick the germ 

 layers at this stage are still spread out on the surface of the large yolk 

 mass. 



Another structure of early origin, as in the lower forms dealt with in the 

 preceding chapters, is the neural plate. When the primitive axis appears, 

 the ectoderm is moderately thickened into a broad band over the axis itself 

 and extending backward over the primitive streak. This thickened ecto- 

 derm represents the rudiment of the nervous system and is known as the 

 neural or medullary plate (Fig. 50). Laterally it gradually becomes thinner 

 and shades off into the non-neural ectoderm. As development proceeds 

 the anterior end of the plate becomes broader, indicating already the heavier 

 brain region. The plate is composed of columnar epithelium possibly 

 pseudo-stratified. When the somites begin to develop, a broad groove the 

 neural groove appears along the center of the plate, thus producing ridges 

 the neural ridges or folds along the sides of the groove. The ridges be- 

 come higher and bend in toward the median line until they meet over the 

 groove. The meeting occurs first in the region of the brain; then fusion 

 occurs and a tube the neural tube is thus formed (Fig. 52). The meeting 

 and fusion gradually progress backward. It is obvious from these conditions 

 that the neural tube in the main is formed from before backward; in fact 

 even when closure has occurred in the brain region, the caudal region ex- 

 hibits still the flat neural plate while the intermediate portion shows all 

 stages between the two extremes. 



Body Form. In Amphioxus and the frog the cylindrical body form of 

 the embryo results from simple elongation of the gastrula. In these forms 

 the yolk is wholly encompassed by the egg cytoplasm, is contained in the 

 entodermal cells of the gastrula, and is wholly enclosed within the embryonic 

 body, being gradually consumed as the organism develops. In the bird 

 only a small portion of the great quantity of yolk is enclosed within the 

 cytoplasm of the mature egg; the cytoplasm is really only a small disk resting 

 upon the yolk. As development proceeds the cytoplasmic disk takes the 





