168 
F. M. BALFOUfl. 
General Conclusions, 
Without attempting to compare at length the develop- 
ment of the spiders with that of other Arthropoda, I propose 
to point out a few features in the development of spiders, 
which appear to show that the Arachnida are undoubtedly 
more closely related to the other Tracheata than to the 
Crustacea. 
The whole history of the formation of the mesoblast is 
very similar to that in insects. The mesoblast in both groups 
is formed by a thickening of the median line of the ventral 
plate (germinal streak). 
In insects there is usually formed a median groove, the 
walls of which become converted into a plate of mesoblast. 
In spiders there is no such groove, but a median keel-like 
thickening of the ventral plate (PI. XX, fig. 11), is very pro- 
bably an homologous structure. The unpaired plate of 
mesoblast formed in both insects and Arachnida is exactly 
similar, and beomes divided, in both groups, into two bands, 
one on each side of the middle line. Such differences as 
there are between Insects and Arachnida sink into insigni- 
ficance compared with the immense differences in the origin 
of the mesoblast between either^ group^ and that in the 
Isopoda, or, still more, the Malacostraca and most Crustacea. 
In most Crustacea we find that the mesoblast is budded off 
from the walls of an invagination, which gives rise to the 
mesenteron. 
In both spiders and Myriopoda, and probably insects, the 
mesoblast is subsequently divided into somites, the lumen of 
which is continued into the limbs. In Crustacea mesoblastic 
somites have not usually been found, though they appear 
occasionally to occur, e.g, Mysis, but they are in no case 
similar to those in the Tracheata. 
In the formation of the alimentary tract, again, the 
differences between the Crustacea and Tracheata are equally 
marked, and the Arachnida agree with the Tracheata. 
There is generally in Crustacea an invagination, which gives 
rise to the mesenteron. In Tracheata this never occurs. 
The proctodseum is usually formed in Crustacea before or, 
at any rate, not later than the stomodceum.^ The reverse is 
true for the Tracheata. In Crustacea the proctodaeum and 
stomodseum, especially the former, are very long, and usually 
give rise to the greater part of the alimentary tract, while 
the mesenteron is usually short. 
* If Grobben’s account of the development of Moina is correct this 
statement must be considered not to be universally true. 
