SOME TEACHINGS ON DEVELOPMENT. 215 
the whole compound organism, and mainly consist of a stomach 
with a long stinging trailer appended ; others have confided to 
them the reproduction of the organism ; whilst all the more 
delicate of the thus modified individuals can shrink for shelter 
under certain other of the members of the colony which have 
degenerated into firm protecting scales. And in that higher type 
of compound organism which is characterised by serial instead 
of budding repetition, whole individual segments are often as 
distinctly set apart for the performance of a special function, 
although from their close union one with another it is generally 
difficult to trace such complete specialisation. The extensively 
found adaptation for prehensile and masticatory purposes ofi-ofie 
or more of the anterior segments in so many of these organisms 
readily comes to mind as an illustration of this. 
But let us revert to the consideration of the sets of cells which 
become segregated for special purposes from the primary blasto- 
dermic layers. Once formed they are found to proceed through 
stages of their own, varying in number with the extent of devel- 
opment of the organs which they compose in different animals. 
In one group of animals it will often be found that one or more 
such segregations have progressed in their functional, and corre- 
spondingly in their structural development much further than 
others. In illustration of this we may notice that in the Yerte- 
brata it is the nervous segregation, in the Arthropoda the mus- 
cular segregation, which above all others has attained the 
greatest development. What mammal, for instance, is capable 
of the tenth part of the activity in proportion to its size, which 
is evinced by the sustained flight of the dragon-fly or the pro- 
digious leap of the flea ! 
As an illustration of the progression in development of a 
special segregation, it will for our present purpose be most 
instructive if we trace out the early stages of formation of the 
nervous system in one of the highest types, and if we compare 
those stages with what we find in animals lower in the scale. 
In the Toad, which we may select as a typical Vertebrate, 
there is at first a uniform layer of cells which is separated at an 
early period from the ectoderm ; then a thickening of this layer 
occurs on either side of the axis of the embryo, extending for- 
wards from the protostome, the two thickenings forming the 
boundaries of a groove which lies between them ; we next find 
the groove becoming roofed over by upward extensions of ecto- 
derm, both from behind the protostome and on either side, and 
thus converted into a canal which is at first open in front and 
communicates behind through the protostome with the alimen- 
tary cavity ; next (or even before the roofing in is completed) we 
observe that the anterior part of the nerve-tube becomes en- 
VOL. XX. NEW^ SEIl. P 
