No. 2.] DEVELOPMENT OF PETROMYZON". 26 1 



These remarkable changes in the character and relations of 

 the pineal body in Petroinyzon would certainly seem to indicate 

 that it has acquired some secondary function of importance. 

 Of its primitive structure as an eye there still remain, as al- 

 ready stated, recognizable traces in the distinctly retina-like 

 appearance of the upper vesicle, and, according to Beard, 

 in the pigmentation, although apparently the lens is not even 

 indicated at any time. But any visual powers the organ may 

 once have possessed are now obviously lost, though it may 

 perhaps be sensitive to light, as is suggested by the fact that 

 the epiphysis is visible through the wall of the head. Never- 

 theless the lamprey does not exhibit a simple degeneration of 

 the organ, such as occurs in most of the higher vertebrates, but 

 rather a transformation of it. The formation of the second 

 epiphysial vesicle and the intimate connection found with the left 

 ganglion habenulae are not known to occur in any other type, 

 and certainly suggest a transformation, and not a mere loss of 

 function. What this secondary and acquired function maybe, if 

 any such exists, it would, in the present state of our knowledge, 

 be idle to conjecture. It is, however, important to emphasize 

 the fact that in Petromyzon, the pineal gland, in some of its 

 stages at least, is more distinctly like an optical organ than in 

 any known vertebrate outside of the lacertilian type. From 

 this it would seem to follow that the pineal eye is an organ 

 which was originally functional in the lower vertebrates, even 

 if we admit Dohrn's hypothesis that the Cyclostomata are the 

 degenerate descendants of fishes which were of a comparatively 

 highly organized type. 



The first formation of the Ganglia Habenulce is thus 

 described by Shipley (37) : " This superior commissure is at 

 first covered with but a few ganglion cells, but these afterwards 

 increase until two bodies are formed, the Ganglia Habenulse. 

 The left one is very small, but the right is a structure of con- 

 siderable size, projecting downwards, and backwards, and reduc- 

 ing the lumen of the fore-brain to a Y-shaped slit. These bodies 

 have been fully described by Ahlborn in the adult; it is interest- 

 ing to note that the curious asymmetry they possess is present 

 from their first appearance." The commissure here referred to 

 passes transversely through the roof of the thalamencephalon 

 immediately in front of the epiphysial rudiment. Ahlborn has 



