The supra-œsophageal ganglia are formed independently of the ventral cords as two thickenings of the procephalic lobes ([fig. 205]). The thickenings of the two lobes are independent, and each of them becomes early marked out by a semicircular groove ([fig. 200] D) running outwards from the upper lip. Each thickening eventually becomes detached from the superficial epiblast, but before this takes place the two grooves become deeper, and on the separation of the ganglia from the epiblast, the cells lining the grooves become involuted and detached from the skin, and form an integral part of the supra-œsophageal ganglia.
At the stage represented in [fig. 201] B the supra-œsophageal ganglia are completely detached from the epiblast, and are constituted of the following parts: (1) A dorsal section formed of two hemispherical lobes, mainly formed of the invaginated lining of the semicircular grooves. The original lumen of the groove is still present on the outer side of these lobes. (2) Two central masses, one for each ganglion, formed of punctiform tissue, and connected by a transverse commissure. (3) A ventral anterior lobe. (4) The original ganglia of the cheliceræ, which form the ventral parts of the ganglia[183].
The later stages in the development of the nervous system have not been worked out.
The development of the nervous system in the Scorpion is almost identical with that in Spiders, but Metschnikoff believes, though without adducing satisfactory evidence, that the median integument between the two nerve cords assists in forming the ventral nerve cord. Grooves are present in the supra-œsophageal ganglia similar to those in Spiders.
The mesoblast. The history of the mesoblast, up to the formation of a ventral plate subjacent to the thickened plate of epiblast, has been already given. The ventral plate is shewn in fig. 203 A. It is seen to be formed mainly of small cells, but some large cells are shewn in the act of passing into it from the yolk. During a considerable section of the subsequent development the mesoblast is confined to the ventral plate.
Fig. 204. Longitudinal section through an embryo of Agelena labyrinthica.
The section is through an embryo of the same age as that represented in fig. 200 C, and is taken slightly to one side of the middle line so as to shew the relation of the mesoblastic somites to the limbs. In the interior are seen the yolk segments and their nuclei.
1‑16. the segments; pr.l. procephalic lobe; do. dorsal integument.