Fig. 195. A fairly advanced embryo of the Scorpion enveloped in its membranes. (After Metschnikoff.)
ch. cheliceræ; pd. pedipalpi; p1-p4. ambulatory appendages; ab. post-abdomen (tail).
By the time that twelve segments are definitely formed, the procephalic region is distinctly bilobed, and in the median groove extending along it the stomodæum has become formed ([fig. 196] A). The cheliceræ (ch) appear as small rudiments on the first post-oral segment, and the nerve cords are distinctly differentiated and ganglionated. In the embryonic state there is one ganglion for each segment. The ganglion in the first segment (that bearing the cheliceræ) is very small, but is undoubtedly post-oral.
At this stage, by a growth in which all the three germinal layers have a share, the yolk is completely closed in by the blastoderm. It is a remarkable fact with only few parallels, and those amongst the Arthropoda, that the blastopore, or point where the embryonic membranes meet in closing in the yolk, is situated on the dorsal surface of the embryo.
The general relations of the embryo at about this stage are shewn in [fig. 195], where the embryo enclosed in its double cellular membrane is seen in a side view. This embryo is about the same age as that seen from the ventral surface in [fig. 196] A.
The general nature of the further changes may easily be gathered from an inspection of [fig. 196] B and C, but a few points may be noted.
An upper lip or labrum is formed as an unpaired organ in the line between the procephalic lobes. The pedipalpi become chelate before becoming jointed, and the cheliceræ also early acquire their characteristic form. Rudimentary appendages appear on the six segments behind the ambulatory legs, five of which are distinctly shewn in [fig. 195]; they persist only on the second segment, where they appear to form the comb-like organs or pectines. The last abdominal segment, i.e. that next the tail, is without provisional appendages. The embryonic tail is divided into six segments including the telson ([fig. 196] C, ab). The lungs (st) are formed by paired invaginations, the walls of which subsequently become plicated, on the four last segments which bear rudimentary limbs, and simultaneously with the disappearance of the rudimentary limbs.
