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MANUAL OF ELEMENTARY ZOOLOGY 



folds, which are continuous in front, and behind enclose 

 the blastopore. On either side of the anterior end of the 

 neural folds appears a thickening which becomes divided 

 by a furrow into a gill plate and a sense plate. The neural 

 folds become closer, rise up, bend over, and meet so 

 as to enclose the neural canal, their union taking place 

 first about the middle of their length (Figs. 372, 373, 377, 

 A). Since they enclose the blastopore, the latter comes 

 to lead from the gut to the neural canal and gives rise 

 to a neurenteric canal, but this soon disappears. The 

 neural canal separates from the epiblast above it, formed 



by the outer walls of 

 the neural folds, whose 

 inner sides become the 

 wall of the neural canal. 

 Before the folds have 

 united in front, the 

 open canal between 

 them is enlarged and 

 then divided into three 

 swellings, the rudiments 

 of the fore-, mid-, and 

 hind-brains. It will be 

 seen that in the frog, 

 as in the lancelet, the 

 central nervous system 

 arises by the sinking in 

 and folding off of a strip 

 of the epidermis of the back. This process is found in all 

 Chordata, and is of the highest importance in the drawing 

 of comparisons between them and other animals. During 

 the formation of the central nervous system the body has 

 been elongating and other structures appearing. Below the 

 blastopore, in the area which it occupied before its contrac- 

 tion, there appears a pit known as the proctodeum, and an 

 opening piercing through from this to the gut forms the 

 anus. From anus to blastopore runs a slight groove, the 

 primitive groove. Above it a knob grows out to form the 

 tadpole's tail. Grooves appear on each gill plate marking 

 out the visceral arches, and upon the first two branchial 

 arches branched external gills grow out. Below the head 



blp. 



Fig. 372. 



-An embryo of 

 a later stage. 



the frog at 



an., Proctodeum (invagination which will form 

 anus); blp., blastopore; g.p., gill plate; 

 n.f., neural fold ; s.p., sense plate. 



Note the primitive groove joining anus to blasto- 

 pore. 



