192 THE PINEAL ORGAN 



that of the muco-cartilage of Ammoccetes, and found a marked similarity, 

 more especially with regard to the disposition of the spaces relative to 

 the matrix. A still further point of agreement which Gaskell emphasized is 

 the relation of the somatic musculature to the head region. The somatic 

 musculature seems to have advanced forward in successive stages of 

 phylogeny and encroached upon the posterior border of the head-shield, 

 causing a gradual absorption of this border on either side of the median 

 hard plate which overlies and protects the brain, thus producing a notch 

 on each side between the tongue-shaped plate which overlies and protects 

 the brain and the lateral cornua. Gaskell further suggested that this plate 

 on account of its position and general relations should receive the same 

 name as the similar plate in the trilobites and be called the " glabella." 

 Since Gaskell 's time the microscopical structure and general archi- 

 tecture of the head-shields of the Ostracodermata has been re-investigated, 

 with the aid of modern methods of technique, by Stensio and Kiaer, and 

 an account of their work will be found in the section on Geological Evidence 

 of the Existence of Median Eyes in Vertebrates, Chapter 23. 



The Pineal Nerve, Chiasma and the Pineal or Habenular Tract. 



The nervus pinealis was first described by Ahlborn (1883) as the 

 stalk of the epiphysis. A point of considerable developmental interest 

 is the relation that the delicate pineal nerve of the adult Petromyzon 

 has to the original hollow stalk of the pineal diverticulum in the embryo. 

 In other words, does the pineal nerve of the mature lamprey correspond 

 in its mode of development to the optic nerve of the lateral eyes of verte- 

 brates ? Or is it developed independently of the stalk of the primary 

 pineal outgrowth ? Also, is there anything in the pineal system of the 

 lamprey comparable with the optic chiasma, optic tracts, and central con- 

 nections of the lateral eyes of vertebrates ? These questions are intimately 

 bound up with that of the development of the two pineal vesicles, the 

 nerve-fibres arising from them and the central connections of these with 

 the habenular ganglia and the anterior and posterior commissures. Accord- 

 ing to Kuppfer (1894) (Fig- I 38, A, p. 196) and Johnstone (Fig. 136), two 

 separate pineal diverticula are developed ; a smaller anterior and a large 

 posterior. Studnicka (1893) quite independently came to the same 

 conclusion, but Kuppfer's figure shows the anterior diverticulum in front 

 of the anterior commissure, a position which throws a grave doubt on 

 the identity of the small recess which he depicts, with the future para- 

 pineal organ (Fig. 22, p. 28). On the other hand, Balfour (4-8 mm. larva 

 of Petromyzon), Dohrn (Fig. 138, B, p. 196), and the well-known figures 

 by Dohrn depicting the early developmental stages of the olfactory and 

 pituitary sacs of Petromyzon, show only one pineal diverticulum. These 



