402 



AN INTRODUCTION TO ZOOLOGY 



By the time the primitive streak has reached its definitive size, 

 a rod of cells, in surface view appearing very similar to the primitive 

 streak, has started to grow forward from the primitive knot. This 

 continues to grow at a slight angle to the line of the primitive streak 

 until it passes just beyond the end of the embryonal shield, and as 

 it is accompanied by mesoderm it pushes forward the hinder limit 

 of the pro-amniotic area. It is often termed the head process, 

 but is more suitably designated the notochordal process since it repre- 

 sents the beginning of the notochord. This process is actually an 

 independent cellular strand growing forward from the knot, between 

 the ectoderm and entoderm and most probably composed of cells 

 derived from the latter layer, although of course at the knot itself 

 it is in contact with all three layers. It should not be^confused 



E.S. 



So 



G.w. 



V.S. 



FIG. 139. Chick. Transverse section of part of blastoderm in region of 

 primitive streak in a chick with six pairs of somites (about twenty- 

 four hours). After Riickert. 



B.I., blood islets; C., coelom ; E., ectoderm; En., entoderm; E.S., embryonal shield; 

 E.T., endothelial tube ; G.W., germ wall ; M., mesoderm ; M.P., medullary plate ; P.S., primitive 

 streak ; So., somatic mesoderm ; Sp., splanchnic mesoderm ; V.S., vascular sinus. 



with the primitive streak in spite of its somewhat similar superficial 

 resemblance, as it is an entirely different structure and it marks 

 definitely the axis of the future embryo. 



As the notochordal process is growing forward an alteration 

 occurs in the ectoderm of the embryonal shield lateral to it. It 

 thickens to form the medullary plate, and this extends not merely 

 along the sides of the notochord itself, but also lateral to the front 

 end of the primitive streak. This plate is indicated in the beginning 

 simply by a thickening of the shield and is flat save for the presence 

 of the dorsal groove, a shallow median groove that brings the floor 

 of the plate into contact with the notochordal process. Soon after 

 its formation the extent of the plate becomes sharply defined by its 

 edges rising up from the general level of the ectoderm to form the 

 neural or medullary folds. When the folds become well developed 



