7O ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 
upper region of the head is formed, as in /usecta and Crustacea, by 
the union of these lobes. 
Rathke’s account of the number of rudimentary post-oral sterna 
would lead one to suppose that in the embryo one sternum is want- 
ing. 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 
embrvonic 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 chelz. 
It would be difficult to obtain a more clear and conclusive proof 
than this, that the chele 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 four- 
teenth 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 Articudata, viz. the sessile eyes, and the 
non-development of one of the pairs of antenne. 
§$ 4. Generalizations regarding the Embryogeny of the Articulata, 
and Morphological Laws based on these. 
From all these facts of development, I deduce the following mor- 
phological laws (some of which have already been enunciated for 
particular classes) for the Asticulata (Lnseeta, 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. It is a 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. 
