THE PINEAL SYSTEM OF AMPHIBIA 229 



layer of the skin is absent or scanty, and the cutaneous glands are also 

 either absent or are reduced in size and number. The spot is variable in 

 size and degree of distinctness in different species and in some is absent 

 altogether. It is well seen in the common frog, Rana temporaria (Fig. 

 163), and also in R. subsaltans, of which a drawing clearly showing the 

 " fleck " was published by Gravenhorst in 1829, although its significance 

 was not appreciated at the time ; and later, when Stieda discovered the 

 vesicle beneath it, he considered it to be glandular in nature, and it 



Fig. 16?. — Head of Frog (Rana temporaria), showing the Parietal Spot 

 which lies over Stieda's Organ and between the Lateral Eyes. (From 



Studnicka.) 



became known as the " frontal gland " (" Stirndruse "). Goette (1873- 

 75), who first recognized the existence of the proximal part or epiphysis 

 in the early stages of development of Bombinator igneus, was also the first 

 to establish the connection of this with the end vesicle and homologize 

 these parts with the pineal organ of other classes of vertebrates, although 

 he was in error with regard to the connection which he believed to exist 

 between the pineal organ and the anterior neuropore (see p. 5). 



Development. — The pineal diverticulum in Rana is at first elliptical in 

 form, but soon becomes tubular, the expanded blind-end growing for- 

 ward while the attached end remains at its site of origin, between the 

 habenular and posterior commissures. A constriction then appears 

 marking off the end vesicle from the proximal part, and as the head 

 enlarges and the end vesicle, which is attached to the skin, is carried 

 forward, a narrow neck appears which becomes elongated and forms the 

 stalk. Later the lumen in the stalk disappears and finally the epithelial 

 wall becomes interrupted, and in the fully developed animal the end vesicle 

 or parietal organ is completely separated from the epiphysis by the roof 

 of the skull, which in living representatives of the order has no parietal 

 foramen. The proximal part, or epiphysis, retains its lumen for a con- 

 siderable period, and this may persist even in the fully grown animal, as 

 in the example shown in Fig. 166, which shows the hollow tubular organ 

 lying on the roof of the diencephalon and extending forward as far as the 

 habenular commissure. The pineal recess lies immediately in front of 



