52 Growth in length of the Vertebrate Embryo 



of the deuterogenetic centre dies out after a time, and the dorsal 

 and dorso-lateral part continues to grow, giving rise to the tail. 



In the Ascidians the conditions are much the same, as shown 

 by Conklin in the development of Cynthia. Here the activity of 

 the growth of the ventral lip is perhaps more marked than in 

 Amphioxus. 



In the Hemichordata the question has not been considered 

 very thoroughly. 



It is well known that no part of their body can be compared 

 with the tail of the higher chordates. 



If we are to compare their condition with that of the higher 

 chordates, we must suppose either that 



(i) The deuterogenetic centre dies out wholly almost as soon 

 as formed, and that all growth is interstitial and protogenetic ; or, 



(ii) The deuterogenetic centre continues active all round the 

 blastopore, which however closes, with the result that the anus is 

 at the extreme end of the body, and that there is no tail. 



The egg of the Balanoglossidae forms a hollow blastula, which 

 invaginates as in Amphioxus and forms a gastrula. The gastrula 

 elongates. The blastopore closes but the anus forms at the spot 

 where the blastopore has closed. An anterior unpaired pouch 

 and two pairs of lateral pouches are given off from the gut which 

 form mesodermic sacs, the five cavities which are known as the 

 proboscis or head cavity, the collar cavities and the trunk cavities. 

 There is no segmentation of the trunk cavities as in Amphioxus 

 although the trunk grows out to a far greater extent. 



This latter is an important point for it is not unreasonable to 

 suppose that segmentation may be due to growth being from a 

 definite terminal growing point. 



If growth in length is produced like the tail of a vertebrate 

 is produced, or like a stem of a tree is produced, it is clear that 

 although median organs may be indefinitely prolonged by this 

 means, and yet retain their original characters, lateral organs 

 cannot be indefinitely prolonged and at the same time continue 

 to act as lateral organs. 



A notochord, or a spinal cord, can be produced indefinitely, 

 and these organs do not show segmentation. 



But a lateral blood-vessel, or a lateral nerve, or a branch of 



