624 A. A. W. HUBREOHT. 



chain, of which the Vertebrate spinal cord is the 'outcome. 

 Like the Nemertean medulla, the Vertebrate spinal cord is 

 median, unpaired, and composed of nerve-cells and nerve- 

 fibres ; like the Nemertean medulla, it is a thickening in a ner- 

 vous plexus, originally wholly epiblastic, of which, among 

 Vertebrates, the Amphibian embryos offer such a striking 

 example. This instructive and suggestive case was known to 

 Remak and Strieker (as the '' Nervenschicht" of the frog em- 

 bryo), it was more carefully studied and elaborately described 

 byGoette'(his "Grundschicht^'' of the epiblast, inhis'Entwicke- 

 lungsgeschichte der Unke^), and it has been again recently 

 brought into the foreground by Baldwin Spencer, in his latest 

 paper on the subject. i The latter author compares the Am- 

 phibian plexus with that of Palseonemertea and Schizone- 

 mertea (loc. cit., p. 134), as had already been done before 

 him by my friend Professor Ray Lankester, with whose sug- 

 gestion I at that time (1880) did not yet venture fully to 

 associate myself. 



The numerous data that have since been accumulated for a 

 direct comparison of Nemertea with lower Vertebrates appear, 

 however, now to fully justify that comparison which was first 

 expressed in a footnote to a former paper of mine.^ There can 

 hardly be any doubt as to the existence, consequent upon 

 natural selection, of a constant tendency in the different 

 component parts of living organisms towards simplification 

 and increased efficiency (Roux's ' Kampf der Theile im Or- 

 ganismus'). This fact enables us to understand the gradual 

 supremacy of the median cord in the Nemertean plexus over 

 the two lateral ones. It seems as if it were mathematically 

 demonstrable that for the delicate adjustment of the impressions 

 from the exterior to the co-ordinated movements thereby occa- 

 sioned, one longitudinal central stem in bilateral, lengthened 

 animals would be more efficacious than two lateral ones. And 

 if we ask if, at the final stage of this struggle for supremacy 



* Baldwin Spencer, "Some notes on the early Development of Rana tem- 

 poraria," 'Quart. Journ. Micr. Sci.,' vol. xxv; SuppL, p. 123, 1885. 



2 'Quart. Journ. Micr. Sci./ vol. xx, 1880, p. 438. 



