PIGMENTATION OF THE PINEAL REGION IN BIRDS 75 



it is believed to function at any rate as a light-percipient organ and possibly 

 be of some use to the animal. On the other hand, both parietal sense- 

 organ and pineal sac are absent in the Crocodilia, although habenular 

 and posterior commissures are present. The pigment, as in other classes 

 of vertebrates, is of two types : (1) epithelial, in the form of fine, dark- 

 brown granules, present in the cylindrical cells of the retina, and in the 

 large, round ganglion cells (pigment balls) and also in the epidermis of the 

 skin, except directly over the parietal scale ; (2) mesodermal, the granules 

 of which are contained in branched cells, around the sheaths of the parietal 

 organ and pineal sac, more especially in relation with the blood-vessels 

 and in the subcutaneous tissue. The details of distribution of the 

 pigment in the cells of the retina will be dealt with later in the description 

 of the pineal eyes of Sphenodon and of different types of lizard, and it will 

 only be necessary here to allude to certain general points : 



1. The variability in the degree and distribution of the pigment in 



different orders within the same class. 



2. The similarity in the distribution of the pigment within the retinal 



cells to that in the eyes of many invertebrates. 



3. The presence of pigment in the wall of the pineal sac or end vesicle 



of the epiphysis in the same situations as in the retina of the 

 parietal sense-organ (" pineal eye "). 



4. The presence of pigment in " accessory parietal organs " in the 



same situation, e.g in Pseudopus Pallasii, with other evidence 

 suggests the primary double nature of the organ — the apical 

 part of one member of the pair being ordinarily suppressed, but 

 occasionally appearing as an incompletely differentiated sense- 

 organ ; this organ, however, possesses structural peculiarities 

 which agree even in such details as the distribution of pigment 

 in the cells of the retina, and render its identification as a 

 parietal sense-organ almost certain. 



Pigmentation of the Pineal Region in Birds 



Klinckowstroem (1892) described a peculiar pigmented outgrowth 

 from the surface of the head, opposite the epiphysis, in certain swimming 

 birds, which he considered to indicate the site of a former parietal spot. 



He found this vestige of a parietal spot in only 12 cases out of 200 

 embryos examined by him. These were in : Sterna hinmdo, the swallow- 

 tern (1 ex.) ; Larus marinus, the black-backed sea-gull (4 ex.) ; Larus 

 canus, the common gull (2 ex.) ; Larus glaucus, the green gull (4 ex.) ; 

 and Anser brachyrhynchus, the pink-footed goose (1 ex.). In the adult 

 birds he found nothing special in the region of the parietal spot. 



The outgrowth consists of a small, dome-shaped projection on the 



