42 



TEXT-BOOK OF EMBRYOLOGY. 



piate of ectodermal cells sinks slightly below the general surface level and 

 becomes demarkated from the surrounding ectoderm. The plate extends 

 from almost the cephalic (anterior) extremity of the gastrula to the dorsal 

 lip of the blastopore and even slightly affects the lateral lips. These cells 

 thus circumscribed constitute the neural plate; in this manner the rudiment 

 of the nervous system appears (Fig. 22, a). The ectoderm bordering the 

 margins of the neural plate becomes elevated above the general surface 

 level to form the neural ridges* These also form a rim around the blastopore. 

 The neural plate then sinks farther below the surface level and at the same 

 time the ridges slide across it toward the mid-dorsal line until they meet 

 and fuse with each other. Thus a roof is made over the neural plate, with a 

 small space between the two structures (Fig. 22, b, c). The median fusion 

 begins some distance in front of the blastopore and from there progresses 

 both forward and backward. The closure is not complete in front for some 



Neuropore 



Primitive segment 

 Coelom (myocoel) 

 Intestine 



Epidermis (ectoderm) 

 Neural tube 



Anterior \ lip cf 

 Posterior / blastopore 



Unsegmented 

 mesoderm 



FIG. 23. From vertical section through Amphioxus embryo with 5 primitive segments. Hatschek. 



time, and the opening thus left is called the neuropore (Fig. 23). The neural 

 ridges close in over the blastopore as they do over the neural plate, so that the 

 blastopore no longer opens to the exterior but into the space between the 

 neural plate and its ectodermal roof (Fig. 23). 



\J Mesoderm Formation. Closely following the appearance of the neural 

 plate in the elongated gastrula, one may observe the rudiment of the middle 

 germ layer and the first indication of the axial structure, the notocord, that 

 gives the name Chordata to the great division of the animal kingdom which 

 includes not only the true vertebrates but also such forms as Amphioxus, 

 Balanoglossus and the Tunicata. In a transverse section of the gastrula, 

 in the roof of the archenteron the entoderm exhibits a change which produces 

 three distinguishable parts. An axial part, lying beneath the center of the 

 neural plate, is the rudiment of the notocord. Two dorso-lateral parts, bi- 

 laterally symmetrical, are the rudiments of the mesoderm (Fig. 22). The 

 notocord rudiment advances to the cephalic extremity of the gastrula, and 

 extends caudally to the blastopore. The mesoderm rudiment reaches from 

 the forward end of the archenteron to the blastoporal region where the two 



