202 



THE PINEAL ORGAN 



the hypothesis of there being a greater or less degree of retrogressive change 

 from a more highly evolved system which existed in the extinct ancestors 

 of the living species to the relics of that system which are left in the fishes 

 of to-day. 



A comparison of the pineal impressions present in fossil types of fish 

 with the conditions found in living representatives of the class (Fig. 132, 

 Chap. 17, p. 182, Fig. 231, Chap. 23, p. 330, and Fig. 49, Chap. 3, p. 73) 

 reveals different stages in the devolution of a system in which paired and 



Fig. 142. — Under Surface of the Head Shield of Cyathaspis (Jaekel), Upper 



Silurian. 

 P. : Impression which is believed to have been produced by median eye (epiphysis). 

 Orb. : Orbital notch for lateral eye. 

 Br. : Impressions supposed to be for branchial sacs. 

 Au. : Impressions thought to have been caused by the auditory organs. 



presumably sensory end-organs existed outside the skull immediately 

 beneath the skin, to the vestigial condition which is found in many of the 

 living adult forms of fish, in which all traces of the bilateral nature seem to 

 have disappeared and nothing but a simple tubular stalk and the proximal 

 part or base persist. There are, however, a few cases in which a double 

 or partially subdivided parietal foramen has been recorded in living 

 types of fish (Cattie), and indications of the bilateral nature of the pineal 

 diverticulum have been recorded in the embryos of several species of 

 fish belonging to different orders (Figs. 149, 150, 151, 152). 



Some interesting observations were made by Locy on the earliest 

 stages of the development of the pineal system in dog-fish embryos. 

 He stated that in the neural plate of an embryo 3 mm. long of Squalus 

 acanthias, before the closure of the plate to form the neural tube, there 

 are three pairs of depressions : namely, an anterior, which gives rise 

 to the optic vesicles of the lateral eyes, and two pairs of accessory de- 

 pressions (Fig. 143), of which the anterior pair, placed immediately 



