ORIGIN AND STRUCTURE OF THE HEAD 



27 



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 

 fore-end of the notochord, originally at least, is lying right 



prost. 



I U m IV V 



Fig. 7 Young stage of development of Amphioxus 



(after Hatschek, 1882, fig. 51). 



br. I : first gill pouch of the left side ("anterior entoderm pocket"). 



prost. prostomium 



I, II, III 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, ha^ 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. I 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 



