PINEAL EYE IN LACERTILIA. 231 



iu the figures on PI. XX, which show the various stages passed 

 through before the highest form of development is reached, 

 and also the various forms as the result of degeneration. Each 

 stage save the earliest ones (1, 2, and 3) which are found in 

 the development of Tunicata and Bufo cinerea, represent 

 the permanent condition of the epiphysis in some living form. 

 The question now arises, is it possible to determine at what 

 period or rather within what group of animals the distal vesicle 

 first became difiPerentiated into a pineal eye. There must 

 clearly have been a period during which the hollow epiphysial 

 evagination was not functioning as an eye, precisely in the 

 same way in which the primary optic vesicles must have existed 

 as hollow outgrowths of the brain before they, in like manner, 

 were differentiated into optic organs ; in fact, the three distinct 

 stages of (1), a hollow bladder-like evagination (fig. 4) ; of (2), 

 a distal vesicle connected by a hollow stalk (fig. 5) to the 

 brain; of (3), a vesicle connected with a solid stalk (fig. 6), 

 must necessarily all have intervened before the final stage 

 (fig. 7) was reached. When in any particular form we find one 

 of these three stages are we to assume that in that given form, 

 and hence in the closely allied members of the same group, the 

 epiphysis has never in its philogenetic history reached a higher 

 stage of development than the one in which it is now present ? 

 Suppose, for example, that we find an animal in which the 

 epiphysis has the form represented in fig. 5, must we take it 

 for granted that in that animal and its ancestors no higher 

 stage of differentiation has ever been reached. Taking the 

 animals in which this particular stage is permanent, we find 

 that they include certain Elasmobranchs together with 

 Cyclodus gigas amongst Lacertilia. Now we have clear 

 evidence that, in the forms from which we must suppose 

 Cyclodus in common with all other lizards to be descended, 

 as well as in its nearest living allies, the epiphysis is developed 

 into a pineal eye. To what conclusion must we come in the 

 case of Elasmobranchs; certainly the non-development of 

 a pineal eye in living examples is no proof whatever that such 

 a structure was not present in its ancestors. It must at once 



