At the pole of the ovum facing the ovarian tube there is formed a germinal disc which undergoes a partial segmentation ([fig. 193] bl). A somewhat saucer-shaped one-layered blastoderm is then formed, which soon becomes thickened in the centre and then divided into two layers. The outer of these is the epiblast, the inner the mesoblast. Beneath the mesoblast there subsequently appear granular cells, which form the commencement of the hypoblast[180].
During the formation of the blastoderm a cellular envelope is formed round the embryo. Its origin is doubtful, though it is regarded by Metschnikoff as probably derived from the blastoderm and homologous with the amnion of Insects. It becomes double in the later stages ([fig. 195]).
Fig. 194. Three surface views of the ventral plate of a developing Scorpion. (After Metschnikoff.)
A. Before segmentation.
B. After five segments have become formed.
C. After the appendages have begun to be formed.
During the differentiation of the three embryonic layers the germinal disc becomes somewhat pyriform, the pointed end being the posterior. At this extremity there is a special thickening which is perhaps equivalent to the primitive cumulus of Spiders. The germinal disc continues gradually to spread over the yolk, but the original pyriform area is thicker than the remainder, and is marked off anteriorly and posteriorly by a shallow furrow. It constitutes a structure corresponding with the ventral plate of other Tracheata. It soon becomes grooved by a shallow longitudinal furrow ([fig. 194] A) which subsequently becomes less distinct. It is then divided by two transverse lines into three parts.[181]
In succeeding stages the anterior of the three parts is clearly marked out as the procephalic lobe, and soon becomes somewhat broader. Fresh segments are added from before backwards, and the whole ventral plate increases rapidly in length ([fig. 194] B).
When ten segments have become formed, appendages appear as paired outgrowths of the nine posterior segments ([fig. 194] C). The second segment bears the pedipalpi, the four succeeding segments the four ambulatory appendages, and the four hindermost segments smaller provisional appendages which subsequently disappear, with the possible exception of the second. The foremost segment, immediately behind the procephalic lobes, is very small, and still without a rudiment of the cheliceræ, which are subsequently formed on it. It would appear from Metschnikoff’s figures to be developed later than the other post-oral segments present at this stage. The still unsegmented tail has become very prominent and makes an angle of 180° with the remainder of the body, over the ventral surface of which it is flexed.