Rabl, Metamerism of the Vertebrate Head. 153 



The vagus is polymerous and its ventral roots are ventral 

 roots of the spinal nerves amalgamated with the head. The 

 prechordal part of the skull is non-vertebral. The visceral 

 arches are regarded homodynamous with ribs. 



Dohrn, by reason of the large number of myotoms which 

 he identifies, regards nasal sac, lense, hypophysis, mouth, thy- 

 roid, auditory sac, etc., as modified gill-clefts. In view of the 

 fact that VanWihje finds 4 segments in the anterior head region 

 and Dohrn 12-15, o'''^ 's relieved to agree with Rabl that there 

 is no mesodermal seginentation' of this region. He requires 

 that a priniitive segment should form a part of the dorsal (and 

 only the dorsal) mesoderm and that it divide into (muscular) 

 niyotom and (osseous) sclerotom portions. 



In the Verhandlungen der Anatomischen Gesellschaft, June, 

 1892, (appearing as Erganzungsheft des Anatomischen Anzeig- 

 ers), Professor Froriep describes anew the so-called neuromers 

 which he had observed in mole embryos, but concludes that 

 they are of no special morphological significance. He also con- 

 cludes that the so-called primary neuromers detected by Kupffer 

 in Triton embryos are simply the results of underlying mesod- 

 erm-somites. 



In the same publication Professor Hatchek discusses meta- 

 merism in Amphio.Kus and Ammocetes. The brain of Amphi- 

 oxus is, in the embryo, a cephalic enlargement of the medullary 

 tube larger than the cord and consists of three cavities, of which 

 the first is the primary prosencephalon. From the base of this 

 vesicle the cavity (infundibulum) is extended to connect with 

 the ciliated sac, to which the olfactory also passes. 



The s^coid portion (mesencephalon) has a narrowed cavity. 

 The third (metencephalon) has a characteristic dorsal expansion 

 or tossa rhomb>idalis. 



The ciliated sac corresponds to hypophysis and olfactory 

 organ of Craniala. In Ammocetes the external opening of the 

 hypojjTiysis is still dorsal and is connected with the olfactory. 



The dorsal root of the spinal nerves, which has no con- 

 nection with the ventral, passes directly to the inner skin, there 

 dividing into a dorsal and ventral branch. Small clusters of 

 ganglion cells occur, especially at the point of bifurcation. The 

 whole length of the nerve from cord to skin is to be regarded as 



