GENESIS OF THE NERVOUS SYSTEM 35 



of karyokinesis in the cells are similar to the 

 phenomena of crystallisation in the mineral world. 

 The mechanical phenomena of karyokinesis remind 

 one of the polarisation which light suffers in 

 crystals of double refraction, and the curious 

 molecular dispositions of the included crystals or 

 microliths. 1 



In the same way as the cellular organism is 

 evolved by successive unfoldings, the morula, 

 blastoid, gastrula, and all the animal world up to 

 man takes form, and I need not treat of them here, 

 as they are facts studied and known, and, albeit 

 not complete, their common descent is indisputable. 

 Neither is it for this book to show how the skin, 

 senses, and the whole cerebro-spinal axis are 

 formed from the ectoderm. 



We will, however, treat briefly of the neurona, 

 which is a newer fact and less known, and above 

 all because it aids directly in the comprehension of 

 these points. 



We can easily understand that if the forms 

 of bodies obey the mechanism of natural forces, 

 the neurona, like all cells and bodies, obeys in 

 its form the same mechanics ; and thus, as every 

 vibration and every colour has on its substantia- 



1 See Light, by Tyndall, p. 89 ; and Cours de Mineralogie, Lapparent, 

 p. 333. 



