Development of the Tail 17 



continues, with the result that the metenteron is increased in 

 length and new tissue is added on to the back, sides, and ventral 

 walls of the animal, and to all organs which at this time are present, 

 and thus the whole embryo, gut, nervous system, notochord, 

 mesoderm, increases in length by the addition of new material 

 posterior to all previously existing tissues. 



The Tail. 



After a certain time however and again as a result of the 

 activity of the deuterogenetic centre, the lips of the blastopore 

 approach and coalesce, and this is followed almost at once by the 

 dying out of the ventral and ventro-lateral parts of that area of 

 cell production. 



The dorsal region and dorso-lateral region however continue 

 their growth for a considerable time longer, but the chief parts 

 with which they are now continuous are the neural tube, the 

 notochord, and the dorsal and dorso-lateral parts of the ectoderm 

 and mesoderm. 



The metenteron is on the whole more ventral, so that there 

 is no great tendency for the gut cavity to be continued backwards. 

 Also none of the ventral or ventro-lateral part of the body wall 

 is continued backwards. Growth in length ceases with regard to 

 these parts. 



In some Amphibians (some Urodela) the blastopore partly 

 closes by the coalescence of its lateral lips, leaving the extreme 

 ventral and the extreme dorsal parts open. The ventral part 

 remains open as the anus after the dying out of the ventral region 

 of the deuterogenetic centre. The dorsal part becomes involved 

 in the folding up of the neural folds which adjoin its anterior and 

 antero-lateral margins and it forms in that way a temporary 

 communication between metenteron and neural tube. This is 

 called the Neur enteric canal. 



In Bana and other Anura the ventral part of the blastopore 

 closes along with the middle part; but the anus forms in the 

 closed area and, as the deuterogenetic area dies out almost 

 immediately, no growth in length occurs here. 



Thus the dorsal part alone continues to grow and to add on 

 more and more segments, so producing the chordate tail, which 



A. v. E. 2 



