SOME TEACHINGS OE DEVELOPMENT. 
213 
fibres, which interlace with those of neighbouring cells, and 
probably serve to convey any impressions received by the cells 
of which they form a part, to deeper lying muscular cells which 
have lost their place in the superficial ectodermal layer and their 
connection with the external medium. 
The nerve-epithelium cells thus formed may themselves 
tend to sink below the general surface of ectoderm. If this 
happens they lose the characteristic shape of epithelium cells, 
and become rounded, with extensions in the direction of the 
nerve-prolongations. In fact, like the muscular tissue, they 
assume a minute structure which is similar to that of the nerve- 
cells and nerves of the higher animals. Nevertheless these 
nerve-cells in the jelly-fishes in no case separate themselves 
from the ectodermal layer. They lie, in fact, between it and the 
muscular layer in those parts in which the latter occurs. 
And the sense-organs in this branch of the animal kingdom 
also show that their essential place of origin is from the ecto- 
derm. For visual organs first make their appearance as patches 
of ectoderm-cells filled with coloured pigment, and some of 
them with nerve-fibre processes connecting, them with adjacent 
nerve-epithelium cells ; auditory organs as ectoderm-cells, con- 
taining crystals in their interior, and similarly connected; and 
olfactory organs as little pits lined by ciliated ectoderm-cells, 
and connected likewise to a nerve-epithelium.^ 
Amongst other animals we observe that in Sagitta also the 
segregation of ectoderm to form a nervous layer is in the first 
instance general and not localised. Eventually this segregation 
becomes accumulated mainly in two situations to form the cephalic 
and abdominal ganglia. In Amphibia, too, the separation of 
ectodermic-cells to form a nervo-sensory layer is at first general. 
But in most animals, e, g. the Earthworm, Euaxes, Ascidia, 
Amphioxus, the segregation in question begins at one part only 
of the edge of the protostome, as is the case with the meso- 
dermic segregation — this situation having been possibly deter- 
mined by the high functional importance of this orifice (assum- 
ing as before with Haeckel that it originally served as a mouth). 
Extending from this situation, the nervous separation would no 
doubt be chiefly guided by the arrangement of the pre-formed 
muscular segregation, and would hence tend to assume a bilateral 
condition such as we see it to possess. 
^ It is the opinion of Professor Claus that the depression which is 
seated above the base of each lithocyst in Aurelia and allied forms of 
Medusm represents an olfactory organ. And the comparative researches 
of l)rs. O. and R. Hertwig have rendered it more than probable that 
the otoliths of the jelly fish even when they appear, as in Aurelia, to be 
connected with entoderm alone, are originally derived from the ecto- 
derm. 
