228 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 



Rathke's account of the number of rudimentaiy 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 Astaciis. Of these, the fom- posterior were widely 

 separated, and lay in the pulmoniferous somites ; while the seven anterior pairs extended 

 only a little way beyond the ambxilatory appendages, and were united into a triangular 

 mass. The anterior of these ganglia were the largest, the posterior tlie smallest. The 

 anterior pair gave off the nerves to the chelte. 



It Avotild be diiScult to obtain a more clear and conclusive proof than this, that 

 the chelaj 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 abdommal somites of the Crustacean ; the aculeated 

 sting corresponds ^^dth 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 antennae. 



§ 4. Generalizations regarding the Embryogeny of the Articulata, and Morphological 



Latos based on these. 



From all these facts of development, I deduce the following morphological laws (some 

 of which have already been envmciated for particular classes) for the Articulata {Insecta, 

 Arachnida, Ch'ustacea) 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 rvidiment. 



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 gi*adually 

 extends tergally so as to form the hasmal wall. 



I?. 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 haemal region of the 

 head. In these cases the top of tlie 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 

 antennae is frequently altogether a1)ove that of the other appendages, so that they appear 

 to be tergal, and not sternal, appendages. 



5. The anterior extremity of the cephalic blastoderm becomes early divided by a 

 median fissure, each lateral portion being a "procephalic lobe." In Insects the line of 

 junction of these procephalic lobes is the epicranial sutui*e. 



I 



