vn. ventral nerve cord; yk. yolk; me.s. mesoblastic somite; ao. aorta.
The somatic layer of mesoblast is converted into the muscles, both of the limbs and trunk, the superficial connective tissue, nervous sheath, etc. It probably also gives rise to the three muscles attached to the suctorial apparatus of the œsophagus.
The heart and aorta are formed as a solid rod of cells of the dorsal mesoblast, before it is distinctly divided into splanchnic and somatic layers. Eventually the central cells of the heart become blood corpuscles, while its walls are constituted of an outer muscular and inner epithelioid layer. It becomes functional, and acquires its valves, arterial branches, etc., by the stage represented in [fig. 201] B.
The history of the mesoblast, more especially of the mesoblastic somites, of the Scorpion is very similar to that in Spiders: their cavity is continued in the same way into the limbs. The general character of the somites in the tail is shewn in [fig. 207]. The caudal aorta is stated by Metschnikoff to be formed from part of the mesenteron, but this is too improbable to be accepted without further confirmation.
The hypoblast and alimentary tract. It has already been stated that the yolk is to be regarded as corresponding to the hypoblast of other types.
Fig. 207. Tail of an advanced embryo of the Scorpion to illustrate the structure of the mesoblastic somites. (After Metschnikoff.)
al. alimentary tract; an.i. anal invagination; ep. epiblast; me.s. mesoblastic somite.
For a considerable period it is composed of the polygonal yolk cells already described and shewn in figs. 203, 204, and 205. The yolk cells divide and become somewhat smaller as development proceeds; but the main products of the division of the yolk nuclei and the protoplasm around them are undoubtedly cells which join the mesoblast ([fig. 203] A). The permanent alimentary tract is formed of three sections, viz. stomodæum, proctodæum, and mesenteron. The stomodæum and proctodæum are both formed before the mesenteron. The stomodæum is formed as an epiblastic pit between the two procephalic lobes ([figs. 200] and [205], st). It becomes deeper, and by the latest stage figured is a deep pit lined by a cuticle and ending blindly. To its hinder section, which forms the suctorial apparatus of the adult, three powerful muscles (a dorsal and two lateral) are attached.