THE PINEAL SYSTEM OF FISHES 205 



vault and the base of the pedicle — compare A, B, C, Fig. 130. This 

 extension of the epiphyseal tube is largely due to the increase in size of 

 the midbrain and hemispheres ; though in some types, e.g. Spinax 

 (Fig. 49, Chap. 3, p. 73), there is a displacement forward, relative to the 

 brain, of the parietal pit in the roof of the skull ; and there is a horizontal 

 portion of the tube the lengthening of which is produced as a result of 

 this displacement. In Spinax also there is a thickened and slightly 

 expanded proximal part of the tube which lies between the pineal recess 

 and the upper margin of the habenular ganglion. This corresponds in 

 position to the epiphysis of mammals, but beyond a slight increase in 

 the thickness and folding of its walls, there is no structural change in 

 this segment and in most species the proximal part passes insensibly into 



Fig. 



1 44 . — Transverse 

 Planes A, B 



IN THE 



A' 

 Sections of the 28-MM. Acanthias Embryo 



WHICH ARE INDICATED IN FlG. 130, B, p. 180. 



Ep. : epiphysis. /. ch. : lamina choroidea. 



FB. : forebrain. MB. : midbrain. 



he. : habenular commissure. v. : velum. 



the stalk, without any abrupt change to mark the transition of the one into 



the other. The relation of the habenular commissure to the habenular 



ganglia and the walls of the midbrain is shown in the transverse sections 



A and B, Fig. 144. The earliest stages of the development of the pineal 



region of Acanthias were studied by Locy (1893-5). He described (see 



p. 203) three paired hemispherical depressions arranged serially one behind 



the other near the outer edge of the unclosed medullary plate. The first 



pair give rise to the optic vesicles. The second and the third pair he 



described as " accessory optic vesicles." With regard to the second pair of 



primary depressions or anterior pair of " accessory optic vesicles," Locy 



states : ' "I have been able to follow the anterior pair step by step 



through a graded series of embryos without having once lost trace of them, 



and to see that they enter the thalamencephalon, and give rise to the pineal 



outgrowth. The posterior pair, which are smaller, are not to be followed 



in this definite way ; they become fainter and I believe they fade away." 



Later he states that " the bulging of the walls to form the midbrain vesicle 



1 W. A. Locy, 1893, Anat. Anz., Bd. IX. 



