462 THE EVOLUTION OF MAN. 



live in the exuded mass, and aid in constructing the mantle. 

 In this way the strong external covering is at length 

 formed, grows thicker and thicker, and in many adult 

 Ascidia constitutes upwards of two-thirds of the entire mass 

 of the body. 123 



The farther development of the individual Ascidian is 

 of no special interest to us, and we will therefore not continue 

 to trace it The most important result, supplied by Onto- 

 genesis, is its perfect agreement with that of the Amphioxus 

 in the earliest and most important stages of its germ- 

 history. It is only after the medullary and intestinal tubes, 

 and, between these, the notochord with its muscles, have 

 been formed, that their development takes different direc- 

 tions. The Amphioxus pursues a steadily progressive course 

 of development, till it entirely resembles the parent-forms 

 of the higher Vertebrates, while the Ascidian, on the con- 

 trary, enters on a course of retrograde metamorphosis, and 

 finally, in the developed state, appears as a very imperfect 

 member of the Worm group. 



Those who again review all the remarkable facts which 

 we have found both in the structure and in the germ- 

 history of the Amphioxus and Ascidian, and who then 

 compare these with the previously ascertained facts of 

 human germ-history, will not think that I have ascribed 

 exaggerated importance to these highly interesting animal 

 forms. For it is now evident that the Amphioxus as the 

 representative of Vertebrates, and the Ascidian as the repre- 

 sentative of Invertebrates, form the bridge which alone can 

 span the deep gulf between the.se two main divisions of the 

 animal kingdom. The fundamental agreement exhibited 

 by the Lancelet and the Ascidian in the first and the most 



