THE DEVELOPMENT OF ADMETUS PUMILIO. 61:] 



were the chief points of resemblance. The chief points of 

 difference are — in Vertebrates the origin of the nenronieres 

 is evidently secondary; nerves issue from the spinal cord only 

 in those places where the vertebral sheath leaves them spaces 

 to pass thi'ough ; in Arachnids the neuromeres appear, in 

 Admetus at least, to be primary in origin, and at the same 

 time to be segmental. This does not point to any funda- 

 mental difference in the whole process ; on the contrary, it is 

 only to be expected that the segmentation which occurs in 

 Arthropods should be more constant and more early in 

 appearance than that of Vertebrates. In these the segmen- 

 tation is disappearing everywhere, and in all systems of 

 organs. 



According to a theory of von Kupffer and others, the 

 sense-organs of the Vertebrates are derived from a hypothetic 

 primitive form of sense-organs, the so-called placods. The 

 placods remain in their least changed form as the " Hiigel- 

 organe der Seitenlinie " of Leydig, and as papillae of the 

 organ of taste. These sense-organs much resemble those 

 just described as building part of the Anlage of the ventral 

 ganglion-cord of Admetus. A similar feature is that the 

 epidermis of the branchial region of Vertebrates gives rise 

 to the epibranchial ganglia, — that is to say, in Vertebrates, as 

 in Arachnids, primitive sense-organs take part in the build- 

 ing up of the ganglia. 



The Sarasins (41) have also pointed out the resemblance of 

 small sense-organs of Helix Waltoni to the "Hiigel- 

 organe" of Ichthophis. Here the sense-organs also seem 

 to take part in the building up of the ganglia. A peculiar 

 coincidence is to be found when we compare the brain of 

 Helix Waltoni with that of Admetus. The Sarasins men- 

 tion that the most anterior portion of the brain of Helix 

 Waltoni consists of very small, darkly stained, and closely 

 packed nuclei, containing no Punktsubstanz and no ganglion- 

 cells; this portion of the brain of Helix Waltoni they 

 call the accessory brain. The process of the development of 

 the accessory brain of Helix Waltoni resembles the same 



