Il6 THE PINEAL ORGAN 



problem is assisted by a general knowledge of the essentials of comparative 

 embryology of invertebrates, in. addition to that of vertebrates. The 

 net result of such a study appears to indicate that the common ancestral 

 stock of living vertebrates and invertebrates must have lived in a very 

 remote geological period ; and although that part of the brain which is 

 connected with the principal sense-organs, along with the alimentary 

 canal and sympathetic nervous system, appears to have an origin which is 

 common to both ; the spinal cord and vertebral column of the vertebrates 

 seem to have been evolved as an entirely new structure which has grown 

 backward in a caudal direction from the head-end of the animal and with 

 its somatic system of nerves and musculature has become incorporated 

 with the primary parts which pre-existed in the pre vertebrate stock. 



If this view is accepted in principle, the possibility of the derivation 

 of the parietal sense-organs or " pineal eyes " of vertebrates from the 

 simple eyes of invertebrates appears to be not only possible, but in our 

 opinion very probable, and any divergence in structure between the two 

 types of eye, are only what might be anticipated in view of the extremely 

 long period of time which has elapsed since the vertebrate stock branched 

 off from the common ancestral stock of the two great Classes. 



The Eyes of Insects 



In the more highly evolved types, such as the dragon fly, the bees, 

 wasps, and ants, or the butterflies and moths, there are two large, upright, 

 lateral eyes of the compound faceted type, and frequently there are also 

 one or two pairs of median eyes of the simple, upright type. These may 

 be fused or separate and are usually small and degenerate (Fig. 20, p. 26). 

 In the wingless insects, Apterygota, compound eyes and ocelli are some- 

 times present and sometimes absent. In the Order Collembola, which 

 includes the spring-tails, compound eyes are never present. Ocelli or 

 simple eyes with a non-faceted cornea are sometimes placed laterally, 

 especially in the larval condition, where they may precede or co-exist 

 with the rudiments of the compound eyes. The facets on the surface of 

 a compound eye are usually hexagonal, and it is stated that in the dragon- 

 fly there may be as many as 28,000. When seen in longitudinal section 

 each ommatidium is found to consist of a cornea-lens formed by a modifica- 

 tion of the cuticle which corresponds to one of the hexagonal facets seen 

 on the surface ; beneath this is a crystalline cone or a group of four 

 crystal cells. In the central axis is a clear, glass-like rod, the rhabdome ; 

 the sensory cells of the retinula end in nerve-fibres which pierce a fenes- 

 trated basement membrane, beneath which is a plexus of nerve-fibres. 

 The retinulae and crystal cones are surrounded and isolated from one 

 another by pigment cells. 



