228 
We will revert to this question and to the many divergent 
views on it in due time (cf. p. 247). 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 ever 
student of zoology. Great is the number of investigators 
that have worked on this subject and the complicated 
problems involved 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. Il (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- 
dly 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 
art of the brain homologous with the supraoesophageal 
ganglia of the Invertebrates, and it is difficult to believe 
there is nct 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 
1) We have seen above that in early stages of development it is. 
quite distinctly recognizable. 
