213 
support for the snout which is used by the animal for 
burying itself in the sand. By this process the anterior end 
of the notochord secondarily soon reaches a considerable 
distance in front of the neuropore. 
Praechordal brain in Craniates. — If presently we turn to 
the head of Craniata, we find here a disposition which 
exhibits a fundamental difference from things as they occur 
in the head of Amphioxus. While in the latter the 
tore-end of the notochord, originally at least, is lying right 
prost, Et En nf 
ZEN ì 
EE 
Er orterd blofsbl 
Er 
ros 
Ge 
Fig. 7 Young stage of development of Amphioxus 
after HATSCHEK, 1882, fig. 51). 
br. 1: first gill pouch of the left side (“anterior entoderm pocket”). 
prost. prostomium 
1, IL, II etc. segments of the soma 
under the neuropore, the whole nervous system being 
epichordal and its anterior end indicating the limit of pro- 
stomium and soma, things are different in Craniates. Here 
part of the brain reaches in front of the anterior end of 
the notochord, being thus praechordal, and if this fore-end 
of the notochord here also marks the limit of prostomium 
and soma, we are induced to suppose that the medullary 
tube must have acquired a forward prolongation and, as 
it were, has annexed part of the epithelium of the prostomium. 
Thus it would be explained that the rudiment of the central 
nervous system reaches up to the anterior end of the embryo, 
thus differing from that of Amphioxus. This is the second 
main principle of my theory. l first, however, was led to 
it along a quite different path, scil. by a comparison of the 
manner in which the eyes originate in Craniates and in worms. 
Optic organs of Acrania. — A comparison of the optic organs 
of Craniates with those of Amphioxus or the Ascidian larvae 
does not take us very far. It is true that in both cases they take 
