356 TJIE PINEAL GLAND. 



This commissure is probably homologous with, and derived from, a 

 commissural band in the roof of the thalamencephalon, placed imme- 

 diately in front of the pineal gland which is well developed in Elas- 

 mobranchii (fig. 254). 



The roof undergoes more complicated changes. It becomes di- 

 vided, on the appearance of the pineal gland as a small papilliform 

 outgrowth (the development of which is dealt with separately), into 

 two regions — a longer anterior in front of the pineal gland and a 

 shorter posterior. The anterior region becomes at an early period 

 excessively thin, and at a later period, when the roof of the thalamen- 

 cephalon is shortened by the approach of the cerebral hemispheres 

 to the mid-brain, it becomes i^pide figs. 250 and 255 chd 3 and 254) 

 considerably folded, while at the same time a vascular plexus is formed 

 in the pia mater above it. On the accomplishment of these changes 

 it is known as the tela choroidea of the third ventricle. 



In the roof of the third ventricle behind the pineal gland there 

 appear in Elasmobranchii, the Sauropsida and Mammalia transverse 

 commissural fibres, forming a structure known as the posterior com- 

 missure, which connects together the two optic thalami. 



The most remarkable organ in the roof of the thalamencephalon is 

 the pineal gland, which is developed in most Vertebrates as a simple 

 papilliform outgrowth of the roof, and is at first composed of cells 

 similar to those of the other parts of the central nervous system (figs. 

 250, 252, 254 and 255 jyn or imi). In the lower Vertebrata it is 

 directed forwards, but in Mammalia, and to some extent in Aves, it 

 is directed backwards. 



In Amphibia it is described by Gotte (No. 296) as being a product of 

 the point where the roof of the brain remains latest attached to the external 

 skin. 



The figure which Gotte gives to prove this does not ajjpear to me fully 

 to bear out his conclusion ; which if true is very important. Although 

 I directed my attention specially to this point, I could find no indication 

 in Elasmobranchii of a process similar to that described by Gotte, and his 

 observations have not as yet been confirmed for other Vertebrates. Gotte 

 compares the pineal gland to the long-persisting pore which leads into the 

 cavity of the brain in tlie embryo of Amphioxus, and we might add the 

 Ascidians, and, should his facts be confirmed, the conclusion he draws from 

 them would appear to be well founded. 



The later stages in the development of the pineal gland in different 

 Vertebrates have not in all cases been fully worked out\ 



In Elasmobranchii the pineal gland becomes in time very long, 

 and extends far forwards over the roof of the cerebral hemispheres 

 (fig. 254 'pn). Its distal extremity dilates somewhat, and in the 

 adult the whole organ forms (Ehlers, No. 337) an elongated tube, 

 enlarged at its free extremit}'^, and opening at its base into the brain. 

 The enlarged extremity may either be lodged in a cavity in the 



1 For a full account of this subject vide Ehlers (No. 337). 



