74 THE PINEAL ORGAN 



Pigmentation of the Pineal Region of Amphibia 



Little has been recorded about the presence of pigment in the pineal 

 organ of Urodeles. Eycleshymer (1892), however, alluded to the presence 

 of pigment in the inner ends of the cells, forming the many-layered inner 

 wall of the hollow epiphysis in Ambly stoma mexicanum, and according to 

 Galeotti (1897), in Proteus anguineus pigment granules are present in its 

 very small pear-shaped epiphysis. The pigment granules are situated 

 in special cells in the neighbourhood of the nucleus, and he states that 

 this is the only place in the brain of Proteus in which pigment is present. 



In the Anura, or tailless amphibians, the allusions to the presence of 

 pigment and its situation with reference to the parietal sense-organ 

 (Stieda's frontal organ) and the epiphysis are much more definite than 

 is the case with the tailed amphibians. Thus, the " parietal spot " was 

 accurately described by Stieda in 1865 : " There is to be seen a slight 

 bulging of the skin on the top of the head, between the lateral eyes. 

 When the skin lying over the organ is removed, the spot is seen to be 

 transparent ; this being principally due to the absence of pigment. The 

 corium surrounding the pigment-free area (parietal cornea) contains an 

 abundance of pigment. A small amount of pigment is, however, present 

 in the epidermis over the frontal organ. The epithelial glands are absent 

 or are reduced in size and number in this region." 



The parietal spot is not equally distinct in different examples of the 

 same species and it is absent in many allied species. Thus, according to 

 Leydig, it is best marked in Rana fusca, whilst in Rana arvalis and Rana 

 agilis only traces are visible and in some specimens it is entirely absent. 



The position of the end vesicle in Bufo is indicated by a clearly 

 defined white spot. In Bombinator the pigment layer and glands are 

 totally absent in the corium over the vesicle ; the epidermis here is, 

 however, strongly arched upwards and its outermost layer is prolonged 

 into a long, deeply pigmented black horn. In Alytes obstetricans, the 

 midwife-frog, the pigment layers of the corium and epidermis over the 

 vesicle are as well developed as in the surrounding skin. 



With regard to pigment in the " frontal organ " (Stirndriise, end- 

 vesicle), Leydig states that the organ contains pigment in Bombinator, 

 but that in frogs the frontal organ is devoid of pigment. 



Pigmentation of the Pineal Organs of Reptiles 



The highest degree of development and differentiation of the parietal 

 sense-organ and " pineal sac " in living animals is preserved in this class, 

 more particularly in Sphenodon and certain lizards, in which the general 

 structure of the organ has been mostly minutely studied, and in which 



