208 THE PINEAL ORGAN 



round, oval, or disc-shaped, and the shape appears to vary within 

 the same species. It is quite likely that some cases in which the end 

 vesicle is in reality heart-shaped have been erroneously described as oval 

 or club-shaped owing to the difficulty of forming a true estimate of its 

 shape when it is seen only in sagittal sections. Unless a reconstruction of 

 the organ is made from a complete series of such sections, an actually 

 heart-shaped organ might easily be regarded as conical. The specimens 

 described by Cattie were drawn from actual dissections of the region, and 

 he gives detailed accounts of their size, relations to membranes, and 

 vascular supply. He also describes in one specimen of Acanthias vulgaris 

 two bilaterally placed parietal foramina separated by a median antero- 

 posterior bridge. Bilateral parietal foramina or impressions are also 

 occasionally found in ancient fossil fishes, e.g. Pholidosteus, Rhinosteus, 

 and Titanichthys (Figs. 225, 226, Chap. 23, p. 325), a circumstance which 

 considerably adds to the significance of their occurrence in living species — 

 which although infrequent should, we think, be regarded as an indication 

 of an inherited trait which is of distinct historical value. 



The end vesicle in cartilaginous fishes is often flattened dorso- 

 ventrally and in some cases the proximal or under surface is thicker than 

 the upper, as in the Cyclostomes. The reverse condition has, however, 

 sometimes been observed, but the similarity in structure of the end 

 vesicle of the fishes generally to that of Cyclostomes may be taken as an 

 indication that in some cases the proximal segment of the end vesicle in 

 fishes, corresponds to the proximal or retinal segment of the " pineal eyes " 

 of the lamprey and the convex distal segment to the " pellucida " or lens. 



Relations of the Pineal Organ to the Roof of the 

 Skull in Selachians 



The end vesicle may lie in a foramen in the anterior part of the carti- 

 laginous roof of the skull and be situated in the median plane between 

 the orbits. Although this foramen is often called the parietal foramen, 

 it must be borne in mind that this region in fishes generally corresponds 

 to the frontal or prefrontal region of the cranial vault (Fig. 132, Chap. 17, 

 p. 182, and Fig. 141, B, p. 201). The term " parietal," signifying wall, is, 

 strickly speaking, correct, but the opening is not necessarily in the region of 

 the parietal bone or bones, as in amphibia and reptiles (Fig. 170, Chap. 19, 

 p. 237, and Fig. 171, A, p. 237). The foramen may be closed superficially 

 by a membrane which is continuous at the margins of the opening with the 

 perichondrium and the dura-mater lining the canal. The end vesicle is 

 usually attached to the walls of the canal by a loose connective tissue, 

 containing vessels, as in Acanthias. In other cases the cartilaginous 



