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purpose. In the individual development Nature cannot manufac- 
ture nerve-cells ad libitum. If there is one place in the universe 
where the doctrine of predestination is justified by actual existence 
in fact, it is in the ontogenetic development of the nervous system. 
In his masterpiece of Annelid development KLEINENBERG !) has 
treated of the phylogeny of nerve structures in a manner which leaves 
little to be desired. The formation of central nerve-cells from peri- 
pheral sense-cells is a truth, which every worker on nerve and nerve- 
cell development ought fo regard as an axiom, for it is one of the 
things which lie at the bottom of so much in nerve development. 
Another axiom is, that a piece of neuro-epithelium, no matter where it 
be, can only grow larger by increase within itself and not by the 
taking up of indifferent epithelium cells, and the endowing them with 
nervous characters. 
Some years ago, in a paper on the branchial sense organs of 
Ichthyopsida (Quart. Journ. Micros. Sci. 1885) I stated that there is 
an actual growth backwards of the lateral line in developing sharks. 
„Ihat is, that the sensory cells which compose the rudiments (An- 
lagen)or she „line: era repeatedly and rapidly divide, and 
in such a manner that the „line“ is increased in length, and pushes 
its way between the indifferent epiblast cells behind it“. The signi- 
ficant bearings of this fact on the morphology of the brain have only 
become obvious to me, since I began the study of the fore-brain region 
in the lower Vertebrates. 
In those forms in which the pallial region is covered by nervous 
matter e. g. (in the Ichthyopsida) the Selachii, Dipnoi and Am- 
phibia, it is such a growth of the neuro-epithelium forming the base, 
front, and sides of the fore-brain in the embryo, which gives rise to 
the thickened nervous roof of the fore-brain. 
It is significant to notice that it is just in these forms among 
the lower Vertebrates that cranial flexure is most pronounced. 
I consider that these two facts stand in connection as cause and 
effect; that it is partly, but not’ entirely, this growth of the original 
basal, frontal and lateral neuro-epithelium of the fore-brain in forward 
and lateral directions towards the neural aspect, which gives rise to 
what we know as cranial flexure?). Figs. 5 to 9 of Eprncer’s memoir 
already cited show these changes sufficiently well. 
1) KLEINENBERG, N., Die Entstehung des Annelids aus der Larve von 
Lopadorhynchus. Zeitschr. Wiss. Zool. Bd. XLIV, 1886, p. 182—224. 
j 2) Cranial flexure is due to several circumstances, these may be 
disclosed in a subsequent essay. 
