EXPLANATION OF THE DIAGRAM. 69 



Non-nervous Adjustments, Unicellular Organisms, and part of 

 the Life-history of the Embryo. Between 9 and 14 is repre- 

 sented the development of Neurility and its passage into 

 Eeflex Action ; the parallel columns within this space are 

 therefore respectively filled with Partly-nervous Adjustments 

 and the beginning of True Nervous Adjustments, Unknown 

 Animals, probably Coelenterata, perhaps extinct, and another 

 portion of the Life-history of the Embryo. I here speak of ^ 

 "unknown animals" because, so far as investigation has 

 hitherto gone, the animals in which nerve-tissue first began 

 to be differentiated have not yet been found. In the lowest 

 animals where this tissue has been found — the Medusae — it 

 appears as already well differentiated. The ganglion cells, 

 however, show in a most unmistakeable manner their parent- 

 age from epithelium — their structure, in fact, often resembling 

 that of modified epithelium more than that of true nerve- 

 cells.* In these structures, therefore (as in the analogous 

 histological elements met with in the embryonic nerve-tissue 

 of higher animals), we have a link which connects true nerve- 

 tissue with its cellular ancestry, and thus it is comparatively 

 immaterial wdiether or not the animals which presented the 

 earlier stages of this histological transition are still in exist- 

 ence. Thus we need not wait to discuss Kleinenberg's view 

 on the " neuro-muscular " cells of Hydra. 



* See Prof. E. A. Schafer on Nervous System of Atirelia Aurita, Phil. 

 Trans., 1878, and Profs. O. and R. Hertwig on Das Nervensystem und die 

 Sinnesorgane der Medusen. 



cyr^ 



