No. 3-] THE VERTEBRATE HEAD. 545 



differentiation. Early in the history of these segments there 

 are seven that clearly belong to the hind-brain, and later there 

 are successively added two more. 



7. Nature, position. — These segments are clearest in the 

 epiblast ; the other layers are slightly affected by the segmental 

 influence; the mesoblast least of all. The segments are seri- 

 ally homologous. 



8. Relations. — They are directly (if not genetically) related 

 to the cranial and spinal nerves. The neural ridges are divided 

 in the same manner as the neural tube. The segments are 

 also directly related to the sense-organs through nerves. 



9. Transformations or modifications. — The modifications 

 are most extreme in the anterior part with the early oblitera- 

 tion of those belonging to the fore- and mid-brains. Those in 

 the hind-brain region are clearly defined for some time after 

 the establishment of the cranial nerves, and then they fade 

 away. 



This is about all that can be said about their transforma- 

 tions, for the modifications have not yet been worked out in 

 detail. 



PART II. — THE SENSE-ORGANS. 



The sense-organs of Vertebrates embrace all those structures 

 that are endowed with special sensibility. They differ from 

 one another mainly in degree of differentiation, and form a 

 series, at the lower end of which are simple sensory papillae, 

 and at the upper end are complex organs like the eye and ear. 

 We recognize two orders of sense-organs — simple generalized 

 ones, the ganglionic sense-organs, and more specialized ones 

 belonging to the so-called five senses. 



The question of the origin and relationship of these sense- 

 organs is full of interest. From the combined results of in- 

 vestiecations on both Invertebrates and Vertebrates it seems 

 altogether probable that the higher sense-organs have been 

 derived from those of a lower order, and, indeed, that they 

 have all been differentiated from a common sensory basis, and 

 are therefore related in a direct way. 



