712 



THE EYE IN EVOLUTION 



1662), which demonstrates figuratively the effect of hght upon the soul lying within 

 the gland. 



It is impossible to overestimate the influence of bene descartes (1596-1650), 

 the great French philosopher, on the development of European thought. In 

 contradistinction to Francis Bacon, the great empiricist who based his philosophy on 

 observed facts, he disregarded the role of experimentation and sought to build a 

 mechanical conception of the universe on mathematical principles. In pure mathe- 

 matics, he invented coordinate geometry, making it algebraic, and developed the 

 conception that mass and time were dimensions as fimdamental as those of space. 

 Finding the intellectual atmosphere of France unsympathetic, he went to Holland 



Par Pin 



Fig. 853. 



Par Pin 

 Fig. 854. 



Figs. 853 to 855. — The Development of 

 THE Median Eye in the Embryo of a 

 Lizard, Lacerta. 



Medial sections through the roof of the 

 diencephalon showing the development 

 of the pineal and parietal organs. Fig. 853 

 in an embryo of 3 mm. ; Fig. 854, 5 mm. ; 

 and Fig. 855, 7 mm. 



E, epidermis ; A'^, neural ectoderm of the 

 roof of the diencephalon. The hatched area 

 represents mesoderm. Pin, the anlage of the 

 pineal organ ; Par, the anlage of the parietal 

 organ ; L, the anlage of the lens (after 

 Novikoff). 



(1628) and there published his two great works, the Discourse on Method (1637) and the 

 Principles of Philosophy (1644), both of which were placed on the list of prohibited 

 books in Rome and Paris (1663). Rejecting the classical view of his time derived from 

 Aristotle that nature was a single system hierarchically ordered with a Deity at the 

 apex, he reasoned that the material vmiverse was a homogeneous mechanical system 

 composed of qualitatively similar activities following quantitative mechanical laws 

 susceptible to mathematical analysis. Alongside this machine-world which included 

 the human body, animals, plants and inorganic natvire, there was a spiritual world 

 in which the body of man alone of all material things participated by virtue of his 

 soul. Ever since his time this dualism of the Cartesian philosophy has permeated 

 European thought ; and although to us today the designation of the pineal body as 

 the meeting place of the two worlds may seem speculative and fanciful, it must be 

 admitted that regarding the function of this organ our ideas are still as nebulous. 



In its most elaborate form the pineal apparatus consists of two parts 

 which arise from the middle of the epiphyseal arch, the most posterior of 



