334 THE PINEAL ORGAN 



spoonbill (Polyodori) (Fig. 50, Chap. 3, p. 73, and Fig. 235). In these the 

 pineal canal does not perforate the roof of the skull and open on the dorsal 

 aspect, but ends blindly, and the pineal organ consists merely of the basal 

 portion and the stalk. The distal end of the stalk is sometimes slightly 

 expanded, but there is no differentiation of a definite eye, such as is seen 

 in Petromyzon, and it is probable that the small pear-shaped vesicle at 

 the distal end of the hollow stalk does not represent the parietal organ. 

 The appearances in the fossil fishes may be accounted for by supposing 

 that : (1) during the ontogenetic development of the cranial roof, the 

 pineal diverticulum was not only surrounded circumferentially by the 

 developing membrane-bone but also covered by it ; or, in other words, 

 the pineal organ never pierced the skull (Fig. 235). Moreover, it may be 



Fig. 235. — Lateral View of the Brain and Pineal Organ of Polyodon folium. 

 The Cranial Capsule has been opened from One Side. (After Garman.) 



Cr. : primordial cranium. Opt. 



Hm. : hemisphere. Pn. 



Olf. : olfactory nerve. St. : 



: optic nerve. 



end-vesicle of pineal organ, 

 stalk of pineal organ. 



inferred that the cases included in this category were : (a) those in which 

 single or paired parietal sense-organs were developed but not fully differen- 

 tiated as an eye or eyes (Fig. 177, p. 248) ; (b) those in which the regression 

 or arrest of development was more pronounced and the pineal apparatus 

 was represented merely by the unpaired base and stalk of the pineal organ 

 (Fig. 49, Chap. 3, p. 73, and Fig. 153, Chap. 18, p. 219). (2) Those in 

 which the growth of the pineal organ was more vigorous and in the develop- 

 ment of the roof of the skull, the stalk of the organ was surrounded on all 

 sides by the developing bone, but the pineal canal not being closed over 

 superficially by a covering layer of bone, the terminal vesicle (or pair of 

 vesicles) was left free to develop outside the skull in the subepidermal 

 areolar tissue. Further, it may be inferred that the cases comprised in 

 this second category may be subdivided into two groups : (a) those in 

 which a more or less fully differentiated eye was developed with an optic 



