74 



TEXT-BOOK OF EMBRYOLOGY. 



and toward the medial line where they finally meet, thus converting the groove 

 into a closed canal or tube, the neural tube (Fig. 56, d; see Chap. XVII). As the 

 ectoderm grows over the neural groove and as the latter becomes transformed 

 into the neural tube, there remains anteriorly an opening from the exterior into 



Anterior (cephalic) end 



Entoderm 



Unsegmented 

 mesoderm 



Archenteron 



Posterior (caudal) end 

 FIG. 57. From horizontal section through Amphioxus embryo with 5 primitive segments; seen from 



dorsal side. Hatschek. 



The communication between the cavities of the primitive segments (ccelom) and the archenteron 

 can be seen in the last 4 segments. 



the neural tube. This is known as the neuropore (Fig. 58) . Caudally, the neural 

 groove extends over the region of the blastopore and as the groove closes over to 

 form the neural tube, it embraces the blastopore which now becomes closed 



Neuropore 



Primitive segment - 

 Coelom (myoeoel) 



Intestine 



Epidermis (ectoderm) 

 Neural tube 



Anterior \ lip of 

 Posterior J blastopore 



Unsegmented 

 mesoderm 



FIG. 58. From vertical section through Amphioxus embryo with 5 primitive segments. Hatschek. 



externally but opens into the neural tube. This opening, which thus connects 

 the neural tube with the intestine, is known as the new enteric canal (Fig. 58), 

 and it is^a rather remarkable fact that while giving rise to no adult organ, it is 

 found without exception in all Vertebrates which have been studied. 



