204 THE PINEAL ORGAN 



each other in the median plane, has not been described, although such have 

 sometimes been shown in a diagram, such as Fig. 136, p. 193, and Fig. 

 138, Chap. 17, p. 196, which is intended to represent in a simple, schematic 

 form the relations which exist either in the embryonic condition or the adult 

 animal in certain types, such as Petromyzon. 



The Pineal Organ of Elasmobranchs 



It will be convenient to describe first the development of the parietal 

 region of the brain of a typical cartilaginous fish, e.g. Squalus acanthias, 

 the spiny dog-fish. The different stages of development of this fish 

 were especially investigated by C. Sedgwick Minot in embryos ranging 

 from ii«5 mm. to 86 mm. in length (Figs. 130, A, B, C, Chap. 17, p. 180). 

 All the component parts of the roof of the diencephalon are present in 

 Acanthias, namely the paraphysal arch, including the paraphysis ; the 

 velum transversum ; the post-velar arch or dorsal sac ; the habenular 

 commissure ; the epiphysis, pars intercalaris, and posterior commissure. 

 Associated with the habenular commissure are, on each side, the habenular 

 ganglia, which as compared with the corresponding ganglia in higher 

 types are of relatively large size in Acanthias (Fig. 130, C). 



In the early stages, e.g. a 15-mm. embryo, the paraphysal arch (Fig. 

 130, B) is a simple, rounded elevation projecting from the roof of the 

 diencephalon immediately in front of the pineal diverticulum. In a 

 sagittal section it might readily be mistaken for an anterior pineal diverti- 

 culum. Its walls are, however, thin, and at a later stage of development 

 it gives rise to the paraphysis. The latter appears at the summit of the 

 arch in a 34-mm. embryo, and at the 70-mm. stage is a simple but slightly 

 folded sac which projects vertically upward immediately in front of the 

 stalk of the pineal diverticulum. The velum transversum hangs down- 

 wards in the ventricular cavity, where it forms a transverse fold the 

 lateral edges of which are continued forward as the choroid plexuses of the 

 ventricle. The dorsal sac is inconspicuous, whereas the epiphysis in the 

 70-mm. embryo is a long, tubular process reaching from the interval 

 between the habenular and posterior commissures to the level of the 

 cranial vault, which in an 86-mm. specimen shows a definite gap, repre- 

 senting the parietal foramen. The distal end of the tube shows a club- 

 shaped expansion, but there is no special differentiation of this part to 

 form a retina and lens corresponding in structure to that of the parietal 

 sense-organ of the lamprey. The growth in length of the epiphyseal 

 tube is in a downward direction relative to the roof of the skull, for the 

 slightly expanded distal end of the tube retains its primary close relation 

 to the ectoderm, while the proximal part is gradually separated from the 

 superficial structures by an increase in depth of the area between the cranial 



