(54) 



central nervous system of the higher vertebrate animals, but evidently 

 he has not seen much of the real structure of the tissue. Besides 

 these statements nothing more is to be found about this part of the 

 brain of amphioxus, and so we seem to be justified in giving an exact 

 description of it here. 



To do this it is necessary to study first of all very thin carefully 

 orientated median sections, as well as frontal and cross sections; the 

 statements by Edinger made it necessary to examine a great many 

 BiELscHOWSKY-preparations to form a correct opinion in this matter; 

 hence we took so long for our research. 



From the very early period at which the infundibular organ is 

 regularly found and the constancy with which it appears, always in 

 exactly the same form and structure, it is evident that it must play 

 a distinct and important part in the animal's life. Already in larvae 

 with only three primary gill clefts the differentiated epithelium is 

 very obvious. Just where the narrow central canal opens into the 

 wider part of the brain-ventricle, we see the ventral limit of the 

 central canal rise slightly and sink again to the former niveau 

 immediately after. This elevation is caused by the cells in the ventral 

 wall growing out into long cylindrical elements, each cell bearing a 

 long hair or ciliuin curving backwards, the cells lying regularly one 

 beside the other. 



It is an important feature in the development of the infundibular 

 organ that the elongation of the cells first shows itself not in the 

 median line but at the left side of the median plane; afterwards the 

 cylindrical cells are also found at the right side. It is only at a 

 much later stage that the long cells fuse in the median line and 

 become one single mass. This and peculiarities in the course of the 

 nerve-fibres springing from the cells in the full-grown organ, point 

 to a bilateral origin of it. 



The cylindrical elongation of the cells is the only change we find. 

 There is no indentation at all of the wall of the brain in front of 

 the organ to be found. 



Already in very young animals we see in well-preserved and well- 

 stained preparations that the cilia of the long cells point backward 

 with a slight curve, the cilia of all the surrounding cells pointing 

 forward, to the anterior neuroporus. 



In older specimens we find the same state of things, but the cells 

 get still more elongated, and the nucleus, now being small and sphe- 

 rical, is lying near the basis of the cell. All cells are directed backwards, 

 that is to say, their free surface being turned craniad. (fig. 8). 



For the topographical relations of the differentiated epithelium to 



