228 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 
Rathke’s account of the number of rudimentary post-oral sterna would lead one to sup- 
pose that in the embryo one sternum is wanting. I believe, however, that the truth is, 
that the sterna of the genital and pectiniferous somites were already so much smaller 
than the rest in the embryos which Rathke chanced to examine, as to be regarded by him 
as one. 
I base this conclusion upon the condition of the nervous system, which consisted of 
eleven pairs of clearly distinguishable post-oral cephalo-thoracic ganglia; that is, of just 
the same number as in an embryonic Astacus. Of these, the four posterior were widely 
separated, and lay in the pulmoniferous somites; while the seven anterior pairs extended 
only a little way beyond the ambulatory appendages, and were united into a triangular 
mass. The anterior of these ganglia were the largest, the posterior the smallest. The 
anterior pair gave off the nerves to the chelæ. 
It would be difficult to obtain a more clear and conclusive proof than this, that 
the chelæ of the Scorpion are the homologues of the mandibles of the Crustacean, and 
that the succeeding somites, as far as the last pulmoniferous one, correspond with the fifth 
to the fourteenth somites, inclusively, of the typical Crustacean. The six succeeding 
somites are the homologues of the six abdominal somites of the Crustacean ; the aculeated 
sting corresponds with the telson; and the only difference presented by the pre-oral 
somites is that common to all air-breathing Articulata, viz. the sessile eyes, and the non- 
development of one of the pairs of antennæ. 
$ 4. Generalizations regarding the Embryogeny of the Articulata, and Morphological 
Laws based on these. 
From all these facts of development, I deduce the following morphological laws (some 
of which have already been enunciated for particular classes) for the Articulata (Insecta, 
Arachnida, Crustacea) generally. 
1. The first-formed rudiment of the embryo corresponds with its sternal surface, or 
with the side upon which the great centres of the nervous system are placed. Itisa 
neural rudiment. 
2. In the thorax and abdomen this neural rudiment grows up on each side towards the 
tergal region, or that on which the great centre of the circulation is placed. 
3. In the Articulate embryo, therefore, the neural wall is formed first, and gradually 
extends tergally so as to form the hæmal wall. d: 
4. The cephalic blastoderm very early undergoes a peculiar flexure, a greater or less 
portion in front of the mandibles being bent up at right angles to the rest, and even in 
many instances extending backwards, so as to constitute the entire hæmal region of the 
head. In these cases the top of the head is in reality a sternal, and not a tergal, surface. 
As a consequence of this flexure, the line of attachment of the bases of the eyes and 
antennæ is frequently altogether above that of the other appendages, so that they appear 
to be tergal, and not sternal, appendages. 
5. The anterior extremity of the cephalie blastoderm becomes early divided by ? 
median fissure, each lateral portion being a “ procephalic lobe.” In Insects the line of 
Junction of these procephalic lobes is the epicranial suture. 
