216 
E. A. SCHAFER. 
larged and subdivided to form the primary parts of the brain, 
and finally, the wall of the tube becomes differentiated to form 
nerve-cells and nerve-fibres. 
If now we compare these stages of development of the nervous 
svstem of the Toad with temporary or permanent conditions of 
the same system in certain animads lower in the scale, we are 
struck at once by the fact that the various stages described are 
more or less represented by those conditions. 
Without again referring in detail to the mode of formation of 
the nervous system as a general segregation from the ectoderm, 
which is met with in the Medusae and in Sagitta — for it might 
perhaps be accounted too bold to attempt a comparison between 
these and the earliest stage, that of general segregation, in the 
Toad — we find that in the Earth-worm the nervous system com- 
mences exactly as in the Toad, in the form of two thickenings of 
the ectoderm which extend forwards from the cup-orifice and form 
the boundaries of a shallow groove. But the development does 
not proceed further in the same way as in the Y ertebrate, for the 
groove is never converted into a canal. 
In the Ascidian we observe as the first stage in the develop- 
ment of the nervous system the formation of a neural groove, 
with its boundaries of thickened ectoderm — so far, as in the 
Earth-worm ; but the development proceeds a stage further. The 
groove becomes gradually roofed in frdm behind forwards, forming 
a tube which long remains open in front and is traceable behind, 
through the protostome, into continuity with the alimentary 
cavity. But ^though the anterior extremity becomes enlarged, 
nevertheless the development of the nervous system in Phallusia 
turns aside from the Vertebrate road, and passes through a series 
of transformations which are special to the Tunicate type. Indeed 
these are of such a nature that the tubular character of the nervous 
system soon becomes no longer recognisable. 
In Amphioxus exactly the same early stages are passed through ; 
there is first the groove, then the enclosure of this to form the 
neural canal, open at first in front, and communicating with the 
alimentary cavity behind. But these apertures before long 
become closed, and the neural canal shut off as a distinct simple 
tube. In this condition it remains permanently, only that the 
walls of the tube become much thickened, so that the ca\dty is 
almost obliterated. Although there is no very apparent de- 
parture from the path which we have seen that the nervous system 
of the Toad takes in its development, nevertheless there is no 
further progression ; the anterior enlargement to form the brain 
never appearing. 
This brief sketch is sufficient to show that the various stages 
in the development of the nervous system of a Vertebrate are 
