THE ORGANS OF THE OUTER GERM-LAYER. 433 



the dermal scutes acquire a special condition and form, and, above all, 

 are transparent. 



In regard to the particular form of the organ, there are essentially 

 three types to be distinguished. 



In many Reptiles, e.g., in Platydactylus, the pineal gland has the 

 same structure as in Sharks : a small peripheral vesicle, which is 



c/i& p et bl x 



Fig. 244. Diagrammatic longitudinal section through the brain of Chameleo vulgaris with the 

 pineal organ, which is separated into three portions, a vesicular, a cord-like, and a tube-like 

 portion, after BALDWIN SPENCER. 



schb, Parietal bone with the foramen parietale ; p, pigment of the integument ; st, cord-like 

 middle portion of the epiphysis ; bl, its vesicular terminal portion ; x, transparent region 

 of the integument ; grh, cerebrum ; sh, optic thalamus ; v*, third ventricle, which is 

 continued upwards into the tube-like initial portion (A) of the epiphysis. 



enclosed in the parietal foramen, is lined with ciliated cylindrical 

 cells, and is connected with the roof of the between-brain by means 

 of a long, hollow stalk. 



In other Reptiles, as in the Chameleon, the organ is differentiated 

 into three portions (fig. 244) : first into a small closed vesicle (6Q, 

 which lies under a transparent scale (x) in the foramen parietale 

 and is lined with ciliated epithelium ; secondly into a solid cord 



28 



