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Transactions of the 
I cannot do better than give you a brief account of what 
Dr. Metschnikoff has done, before passing to the more general con- 
sideration of the subject which 1 have in view. 
According to the observations of that author, the first trace of 
the embryo scorpion is a longitudinal groove in the blastoderm : 
this groove afterwards disappears for a time, but subsequently 
becomes visible again as a deep groove, on either side of which the 
nerve gangha of the ventral chain appear as thickenings of the 
external layer of the blastoderm. Nerve fibres are next seen pene- 
trating the median layer, or mesoblast. The mesoblast splits, as in 
vertebrates, to form the body cavity, its inner layer becoming incor- 
porated with the inner or visceral layer to form the alimentary 
canal. 
In connection with these observations, the remarkable internal 
skin growths which support the principal nerve centres of insects 
and crustaceans are of the highest significance ; as it is quite clear 
that they are the highly-developed remains of the primitive fold in 
which the nervous system is formed. Fig. 4 is a section of one 
from the thorax of the fly, and it shows the close relation of the 
nervous and cutaneous systems. 
One of the most striking circumstances in the development of 
the nervous system is the bifurcation of the neural groove and chain 
at its anterior or cephalic end, so that it embraces the mouth and 
pharynx in a kind of fork, the anterior extremity of which gives 
origin to the great precesophageal or prestomal ganglia. 
The ganglia of the Articulata are chiefly developed at the bases of 
the lateral appendages ; the rudimentary legs, antennae, &c., may be 
conveniently divided into prestomal and poststomal ganglia. The 
prestomal nerve centres, like the brain of the vertebrate embryo, 
make a haemal curve, which is most decided in the most highly 
developed forms, both in the Crustacea and the Insecta. The 
haemal curvature of these nerve centres was pointed out by Professor 
Huxley as early as 1857 in the following terms : — “ It is exceedingly 
interesting to remark the correspondence between the embryonic 
structure of the head of Mysis (and, I may add, of the other Articu- 
lata) and that of the vertebrate embryo. The procephalic processes 
resemble in a remarkable manner the lateral cranial processes of the 
vertebrate embryo ; and the cephalic flexure of the crustacean, or 
insect, has its analogue, if not its liomologue, in the cranio-facial 
inflection of the higher Vertebrata.” * 
Dr. Weissmann, in his ‘ Embryology of the Diptera,’ has 
described the same primitive ventral groove in the early embryo of 
the fly, and my own observations on the development of the blow-fly 
not only confirm Dr. Weissmann’s, but they go far to establish the 
fact that in the highest and most modified forms of the Insecta the 
development of the nervous system takes place in the serous layer 
* lectures on Gen. Zool., ‘Med. Times and Gaz.,’ 1857, p. 638. 
