— 18 — 



though somewhat modified, having in part become discon- 

 tinuous. The columnal layer, „Saulenschicht", could be 

 compared to the „ surface -pits"; but it, of course, no 

 longer forms a continuous layer in Arachnids as it does 

 in Vertebrates. The „Mittelzone" can be compared, as 

 regards position and structure, to the nuclei-groups in the 

 developing ventral ganglion-cord of Admetus, and the 

 „Randschleier u , which gives rise to the white matter in 

 Vertebrates, is evidently equivalent to the commissural 

 subtance, or „Punktsubstanz u of the brain of Admetus, 

 also as regards position and structure. 



The ganglion cells of Admetus are almost always to 

 be found in the depth of the grey matter, just below the 

 Punktsubstanz or white matter, as is also the rule with 

 Vertebrates. In the latter all nerves proceed out of the 

 white subtance. Even this has an analogy in Arachnids, 

 as all the nerves I met with in my cuts left the ventral 

 ganglion-cord just above the grey matter, at places where 

 the nucleiferous covering of the „Punksubstanz u was very 

 thin. These were the chief points of resemblance. The 

 chief points of difference are : — In Vertebrates the 

 origin of the neuromeres is evidently secondary; nerves 

 issue from the spinal cord only in those places where the 

 vertebral sheath leaves them spaces to pass through; in 

 Arachnids the neuromeres appear, in Admetus at least, 

 to be primary in origin, and at the same time to be seg- 

 mental. This does not point to any fundamental 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 segmentation 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 hypo- 

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

 The placods remain in their least changed form as the 

 "Hiigelorgane 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 



