EARLY ORGANOGENY 127 



the general elongation of the entire embryo and its contained archen- 

 teron. At this stage, when the primary nervous structures are being 

 formed, the embryo is known as a neurula. 



The medullary or neural plate extends from the dorsal lip of the 

 blastopore to the anterior limit of the developing embryo, where it 

 appears somewhat rounded in contour. The elevated neural folds 

 are therefore continuous around the margins of this thickened medul- 

 lary plate, the anterior junction of the folds being designated as the 

 transverse neural fold to distinguish it from the paired, extensive and 

 more posterior lateral neural folds. This transverse neural fold repre- 

 sents, then, the anterior extremity of the developing brain. The 

 regions of the lateral neural folds represent the posterior parts of 

 the brain and the spinal cord levels. 



These lateral neural folds move toward each other and first make 

 contact at a point slightly anterior to the center of the original medul- 

 lary plate, a region which will be identified later as the level of the 

 medulla of the brain. From this initial point of contact the neural folds 

 come together and fuse in both an anterior and a posterior direction, 

 thus converting a groove into a closed canal, the medullary (neural) 

 tube or neurocoel. Obviously, fusion will occur last at the extremities, 

 the anterior one being called the anterior neuropore and the posterior 

 one the blastopore. At the posterior end the medullary (neural) folds 

 merge into the sides of the blastopore. As the folds meet they cover 

 over this blastopore and the enteron is therefore no longer opened 

 to the exterior (by way of the blastopore) but into the posterior end of 

 the neurocoel. The original blastopore then becomes a temporary 

 tube-like connection between the gut and the nervous system, known 

 as the neurenteric canal. 



As the neural folds fuse and the neurocoel becomes constricted 

 off from the dorsal ectoderm, the latter becomes a continuous sheet 

 of cells above the mid-dorsal line. The enclosed canal, lined with 

 ciliated and pigmented ectoderm, is the neural canal or neurocoel 

 which is found as the much-reduced central canal of the spinal cord 

 and brain of the adult frog. 



Slightly ventral to the anterior end of the closing neural folds there 

 appears a semicircular elevated ridge of ectoderm, the two exten- 

 sions of the elevation merging with the lateral limits of the transverse 

 neural fold. This is called the sense plate which contains the material of 

 the fifth and seventh cranial nerve ganglia. As the neural folds come to- 



