Sorensen, Study of Epiphysis and Roof of Dicnceplialon. 39 



further evidence is wanted to show that the pineal in isolated 

 cases is found in a greatly stunted condition or perhaps entirely 

 wanting. 



Hertwig (ja-'g2) in his embryology made the following 

 statements concerning the pineal : With the exception of Am- 

 phioxus lanceolatus the pineal gland is not wanting in verte- 

 brates. It is always formed in the same way. According to 

 Ehlers (18—78) the pineal process attains in selachians an unus- 

 ual length. Its closed end swells into a vesicle which pene- 

 trates the cranial capsule and extends out to the dermal surface. 

 In many selachians, such as Acanthias and Raja, the vesicular 

 end is enclosed in a canal of the cranial capsule itself; in others 

 it lies between the cranial capsule and the conarium. In regard 

 to the form of the organ there are three types ; (1) In some 

 reptiles, e. g. Platydactylus, the pineal has the structure found 

 in sharks, i. e., a small spherical vesicle enclosed in the parietal 

 foramen, lined with ciliated, cylindrical cells, and connected 

 with the roof of the diencephalon by a long hollow stalk. (2) 

 In other reptiles, e. g. , Chameleon, the organ is differentiated 

 into three parts ; first, a small closed vesicle under a transpar- 

 ent scale, in the foramen parietale, lined with ciliated epithe- 

 lium ; second, a solid cord consisting of fibres and spindle- 

 shaped cells, bearing a certain resemblance to the embryonic 

 optic nerve ; third, a hollow, funnel-shaped projection which 

 here and there exhibits sac-like projections or enlargements. (3) 

 In a third division of reptiles, e. g., Hatteria, Monitor, blind 

 worms and lizards, the parietal organ resembles the eye of ver- 

 tebrates. The portion of the wall of the organ lying next to 

 the surface of the body has been transformed into a lens-like 

 structure. The part of the wall lying opposite the latter and 

 continuous with the fibrous cord has been converted into a 

 retina-like structure. The formation of the lens is due to the 

 transformation of the epithelial cells into cylindrical cells and 

 uninucleated fibres producing an elevation, the convex surface 

 of which projects into the cavity of the vesicle. The posterior 

 portion of the epithelial cells is separated into different layers, 

 the innermost distinguished by the abundance of pigment. Be- 



