Fig. 213. Two sections of a young Elasmobranch embryo, to shew the mesoblast split off as two lateral masses from the hypoblast.
mg. medullary groove; ep. epiblast; m. mesoblast; hy. hypoblast; n.al. cells formed around the nuclei of the yolk which have entered the hypoblast.
6. The mesoblast is derived from both germinal layers.
Tracheata.—Araneina ([fig. 214]). It is derived partly from cells split off from the epiblast and partly from the yolk-cells; but it is probable that the statement that the mesoblast is derived from both the germinal layers is only formally accurate; and that the derivation of part of the mesoblast from the yolk-cells is not to be interpreted as a derivation from the hypoblast.
Fig. 214. Section through an embryo of Agelena labyrinthica.
The section is represented with the ventral plate upwards. In the ventral plate is seen a keel-like thickening, which gives rise to the main mass of the mesoblast.
yk. yolk divided into large polygonal cells, in several of which are nuclei.
Amniota.—The derivation of the mesoblast of the Amniota from both the primary germinal layers is without doubt a secondary process.
The conclusions to be drawn from the above summary are by no means such as might have been anticipated. The analogy of the Cœlenterata would lead us to expect that the mesoblast would be derived partly from the epiblast and partly from the hypoblast. Such, however, is not for the most part the case, though more complete investigations may shew that there are a greater number of instances in which the mesoblast has a mixed origin than might be supposed from the above summary.