Johnston, Morphology of the Head. 233 



in the trunk region find their exact counterparts in the pahngen- 

 etic and also in the coenogenetic regions of the head, although 

 with various modifications in different classes of vertebrates. 

 There is good reason for thinking that the special sense organs 

 of the head,' their nerves and brain centers, are modifications of 

 one or other of these fundamental functional divisions and not 

 in any sense new structures. The so-called palingenetic head 

 does not havq distinct boundaries and is not constituted of dis- 

 tinct and characteristic structures. 



Recently Froriep (33, 34) has stated his objections to the 

 theory of the palingenetic head, insisting especially that it is 

 false to suppose that there was first developed a primordial head 

 and that this was then modified by various processes of reduc- 

 tion, shifting and dovetailing by which the coenogenetic head 

 was added to the palingenetic. The triple division of the body 

 which he adopts in common with v. Wijhe rests on a scarcely 

 better foundation. The regions are ill-defined and the distinc- 

 tions between them are traceable to processes which are still in- 

 dicated in the ontogeny of vertebrates so that the divisions can 

 not be regarded as primitive. The achordate and caducichor- 

 date regions are specialized through the reduction of the meso- 

 derm and this is due to the development of the definitive mouth 

 (neostoma) and the reduction of the paleostoma. 



i^. The morphology of the visual organs. 



It is important in approaching the subject of segmentation 

 in the rostral part of the head to have in mind all the structures 

 which have segmental significance. The entoderm and meso- 

 derm of this region have been discussed above. The anterior 

 head cavities of Platt and a median prolongation of entoderm 

 are the only structures of segmental value afforded by these two 

 germ layers. When we turn to the ectoderm and brain we find 

 in addition to the two so-called nerves of special sense, the op- 

 tic and olfactory, the nasal epithelium itself and a placode which 

 comes to form the lens, two vestigeal nerves, the N. thalamicus 

 of Platt and the N. olfactorius accessorius of Pinkus and 

 LocY, and two epiphysial organs, not to mention several epi- 



