42 THE ANCESTRY OF VERTEBRATES 



We will revert to this question and to the many divergent 

 views on it in due time (cf. p. 61). The difference in the 

 character of the praechordal part of the head, with the 

 fore-brain and the olfactory and optic organs and their 

 nerves, from the epichordal part, which in several respects 

 shows traces of a former segmentation like that of the 

 trunk or like that of Amphioxus, are known to every 

 student of zoology. Great is the number of investigators 

 that have worked on this subject and the complicated 

 problems ''nvolved in it. To these, often troublesome, in- 

 vestigations the theory propounded here supplies a phylo- 

 genetic basis which makes us understand the significance 

 of the unsegmented anterior part of the body and its 

 relation to the segmented soma. It is again BALFOUR (1881) 

 who seems to have anticipated this result when he writes 

 in his Treatise on Comparative Embryology Vol. II (p. 260) : 

 "In Arthropods and Chaetopods there is a very distinct ele- 

 ment in the head known as the precephalic lobe in the 

 case of Arthropods, and the praeoral lobe in that of Chae- 

 topods; and this lobe is especially characterized by the 

 fact, that the supraoesophageal ganglia and optic organs are 

 formed as differentiations of part of the epiblast covering it. 

 Is such an element to be recognized in the head of the 

 Chordata? From a superficial examination of Amphioxus 

 the answer would undoubtedly be no; but then it has to 

 be born in mind that Amphioxus, in correlation with its 

 habit of burying itself in sand, is especially degenerate in 

 the development of its sense-organs; so that it is not dif- 

 ficult to believe that its praeoral lobe may have become 

 so reduced as not to be recognizable ^). In the true Verte- 

 brata there is a portion of the head which has undoubt- 

 edly many features of the praeoral lobe in the types 

 already alluded to, viz. the part containing the cerebral 

 hemispheres and the thalamencephalon. If there is any 

 part of the brain homologous with the supraoesophageal 

 ganglia of the Invertebrates, and it is difficult to believe 

 there is fnct such a part, it must be part of, or contain, 

 the forebrain. The forebrain resembles the supraoesophageal 

 ganglia in being intimately connected in its development 



^) We have seen above that in early stages of development it is 

 quite distinctly recognizable. 



