THE ORGANS OF THE OUTER GERM-LAYER. 435 



projects into the cavity of the vesicle. In the posterior portion the 

 epithelial cells are separated into different layers, the innermost of 

 which is distinguished by the abundance of its pigment. Between 

 the pigmented cells there are imbedded others, which can be compared 

 to the rods of the visual cells in the paired eyes of Vertebrates, 

 and which appear to be in connection below with nerve- fibres. 



Those investigators who, like RABL-RUCKHARD, AHLBORN, 

 SPENCER, and others, have studied the pineal gland, are of opinion 

 that the pineal body 'must be considered as an unpaired parietal eye, 

 which in many classes, for example in Reptiles, appears to be tolerably 

 well preserved, but in most Vertebrates is in process of degeneration. 



That we have to do in Reptiles with an organ which reacts under 

 the influence of light, does not appear improbable, when one takes 

 into consideration that, owing to the transparency of the dermal 

 scutes at the place in the skull where the parietal foramen is 

 located, rays of light are here able to penetrate through the integu- 

 ment. The presence of a lens-like body and pigment is also 

 favorable to this view. But whether the organ serves for sight, 

 or only for the transmission of sensations of warmth, whether, 

 consequently, it is more an organ for the perception of warmth than 

 an eye, must for the present remain undecided. It is still more 

 an open question whether this organ of warmth is a structure 

 which has been developed as a special modification of the epiphysis 

 of Reptiles alone, as the auditory sac, for example, has been 

 developed in the tail of the Crustacean Mysis, or whether it 

 represents a structure originally common to all Vertebrates. In the 

 latter case processes of degeneration must be assumed to be wide- 

 spread, for up to the present time nothing like the condition in 

 Reptiles has been found in other Vertebrates. 



In Birds and Mammals the pineal process undergoes metamor- 

 phoses which give rise to an organ of a glandular, follicular structure. 



In Birds (fig. 246) it never attains such great length as in 

 Selachians and Reptiles. At a certain stage it sends out from its 

 surface into the surrounding vascular connective tissue cellular out- 

 growths, which increase in number by means of budding and finally 

 break up into numerous small follicles (fig. 246 f). These consist of 

 several layers of cells, the outermost being small, spherical elements, 

 the innermost cylindrical ciliated cells. The proximal portion of the 

 pineal process does not become involved in the follicular metamor- 

 phosis and persists as a funnel-shaped outfolding of the roof of the 

 between-brain ; the individual follicular vesicles constricted off from 



