222 . HOVEY JORDAN 



factory silver preparations (Bielschowsky, Ranson) the fiber and 

 its cells of origin remained unstained. 



In the brook-trout the beginning of the development of the 

 'sub-commissural organ/ and likewise of Reissner's fiber, is 

 nearly contemporaneous with the appearance of the first fibers 

 of the posterior commissure. As seen in parasagittal sections 

 taken near the median plane (fig. A), the organ is at first a small 

 region of slightly thickened epithelium, which, with the enlarge- 

 ment of the posterior commissure, protrudes more and more 

 into the brain lumen from the anterior portion of the roof of 

 the midbrain. With further development and the addition of 

 more commissural fibers the protrusion becomes more prominent 

 and at length has the appearance of a fold of the epithelium, the 

 deep surfaces of the two layers constituting the fold being 

 separated by only a thin layer of connective tissue. In sagittal 

 sections it now has the form of a tongue-like projection extending 

 into the lumen of the brain and making an angle of about 45° 

 ^vith the long axis of the neural tube. At a later stage this tongue- 

 like region is more and more elongated in a caudal direction, and 

 "the angle made by its axis with that of the neural tube decreases 

 (figs. 1 to 3). 



The histological character of the ' sub-commissural organ' is 

 established at an early stage (twenty days) , when probably every 

 one of the ependymal cells sends a fibril into the brain ventricle. 

 The number of these cells increases somewhat as development 

 proceeds, but it is fairly constant from about the fiftieth day 

 after fertilization, there being approximately fifty to sixty cells 

 in a single sagittal section 5 m thick. After entering the lumen, 

 most of the fibrillar processes from these cells converge into 

 larger fibrils. At about this time several cells, which are prob- 

 ably ependymal, detaching themselves from the roof of the 

 brain, enter the lumen (fig. A, cl R.) and become elongated in 

 an anteroposterior direction. Here they seem to fuse gradually 

 and completely with the anterior portion of Reissner's fiber 

 (fig. 1). This detachment of ependymal cells from the roof of 

 the brain appears to be followed by a similar activity in the wall 



