THE PINEAL GLAND 297 



The work of Locy on the elasmobranch genus Acanthias 

 is perhaps not so convincing as that of Hill on the teleosts 

 and Anna, and requires confirmation. If his conclusions are 

 correct we can recognise on the yet unclosed neural plate 

 two pairs of symmetrically developed " accessory optic vesicles " 

 behind the ordinary optic vesicles, with which they would 

 appear to be serially homologous, and the pineal organ is 

 said to be derived from the first pair. Other observers, 

 however, describe the first appearance of the pineal organ 

 in elasmobranchs as a median unpaired outgrowth, but it is 

 not impossible that this may be preceded by the condition 

 described by Locy. 



The existence of a conspicuous parietal foramen in Dolicho- 

 soma, Melancrpeton and other extinct labyrinthodont genera 

 has long been regarded as indicating the possession by these 

 forms of a well-developed pineal sense-organ, but in all living 

 Amphibia the parietal foramen has disappeared and the pineal 

 organ has lost its eye-like character. In living Urodela and 

 Apoda only a single pineal organ occurs, at any rate in the 

 adult, and even this remains in a vestigial condition, apparently 

 representing the proximal portion of the stalk of a more fully 

 developed pineal organ such as occurs in fishes. It has the 

 form of a small sac lying close upon the roof of the thalam- 

 encephalon and is less developed than in almost any other 

 vertebrate group. In the Anura, on the other hand, the single 

 pineal organ of the adult is differentiated into a proximal, sac- 

 like " epiphysis," comparable to the remnant of the organ in 

 Urodela and Apoda, and a distal vesicular " Stirndriise," which 

 lies altogether outside the cranium, and becomes more or less 

 completely separated from the proximal portion by the closure 

 of the cranial roof between them. The " Stirndriise " is an 

 almost spherical sac with cellular walls, of which the lower 

 is much thicker than the upper. It is probably an extremely 

 degenerate pineal sense-organ, and, as in the case of the 

 corresponding organs in the lamprey, its position is indicated 

 externally (in the common frog) by a " Scheitelfleck," clearly 

 visible in the living animal as a small unpigmented spot lying 

 on the top of the head between the paired eyes. Here again 

 it is stated that a bundle of nerve-fibres runs from the dorsal 

 commissure through the dorsal or posterior wall of the proximal 

 portion of the pineal organ (" epiphysis "), which latter contains 



