540 
J. S. KINGSLEY. 
completely closed by the pressure of the surrounding viscera 
and to enable them to open readily for the inspiration of air on 
the relaxation of the pressure some elastic element is needed. 
This is supplied at first by scattered chitinous rods or thicken- 
ings ; next, these are arranged transversely to the tube, and, 
lastly, the rods become united to form the spiral. All of these 
stages can be seen in the adult Araneinse. Leydig^s observa- 
tions ( 78 , p. 265) on the respiratory apparatus of the Onis- 
cidse are suggestive in connection with the origin of tracheae. 
They will be referred to again. 
Nervous System. 
Not until the mesoblast has become divided into the two 
bands mentioned above do we find any trace of the nervous 
system. Its first appearance is as two longitudinal epiblastic 
thickenings (fig. 47), one on either side of the median line. 
There is no external neural groove, but in its stead one on the 
inner surface of the cord. This of course is a variation from 
the normal of but minor importance, and doubtless arises from 
the fact that the egg fills its envelope so completely that an 
inward bending is impossible. In either band the arrangement 
of the cells show that a rapid proliferation is taking place, the 
result being a broad band on either side, the inner portions of 
which are to form the neural cord. At first, as is shown by 
longitudinal sections, this neurulation is continuous, there being 
no differentiation into commissures (connectives) 1 and ganglia 
until a later date. 
The separation of the neural cords from the epiblast takes 
place at first in front and progresses more and more slowly 
posteriorly (fig. 29). The separation of the commissural 
portions is effected before that of the ganglionic areas, and in 
the latter the cleavage takes place first in the median line 
(fig. 25), even while in the lateral areas the epiblast is still 
1 I follow Professor Lankester and others in restricting the use of the 
term commissure to the cords whcih serve to unite the ganglia of the same 
pair, and employing for those which unite the ganglia of a side the term 
connective. 
