DEVELOPMENT OF THE PINEAL SYSTEM OF BIRDS 293 



on the surface of the organ into a thin, fibrous capsule outside which 

 are a number of dilated capillary vessels and small venous sinuses. A 

 secondary stalk has developed at its base and its lumen has already become 

 constricted, but still remains in communication with the third ventricle. 

 At the eighth day the superior commissure has made its appearance and 

 is connected on each side with the habenular ganglion. The posterior 

 commissure has now become folded and forms a rounded cord immediately 

 behind the pineal recess. 



The development of the superior commissure in birds was specially 

 studied by Cameron in 1904. Owing to its late appearance, the com- 



P.V.A. 



Fig. 200. — Median Longitudinal Section of the Pineal Diverticulum of 

 a Chick Embryo (fourth day). The Apex is directed forward. (R. J. G.) 



P. Co. : posterior commissure. P.V.A. : post-velar arch. 



P.D. : pineal diverticulum. V.F. : velar fold. 



P.I.P. : posterior intercalary plate. V . III. : third ventricle. 



missure was thought by authors writing previous to this date to have 

 been absent in Aves. Cameron, however, demonstrated its presence in 

 a 9-day chick embryo. He states that " the nerve-fibres are found to 

 emerge from the ganglion habenulee — from the cells of which they take 

 origin. They pass backwards for a considerable distance (histologically 

 speaking) in the lateral wall of the thalamencephalon, and then cross the 

 mesial plane immediately in front of the epiphysial opening, and pass 

 to the ganglion habenulae of the opposite side, in which they appear to 

 end. Therefore, although both the ganglia and nerve-fibres appear early 

 (by the 5th day), it takes the latter some time to grow backwards in order 

 to cross the median plane. It is thus obvious that they will appear last 

 of all at the latter position. This explains why it is not until the 8th or 

 9th day that they are seen in vertical mesial sections of the same region." 



