Fig. 206. Epibolic gastrula of Bonellia. (After Spengel.)
A. Stage when the four hypoblast cells are nearly enclosed.
B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.
ep. epiblast; me. mesoblast; bl. blastopore.
The nervous system, although imbedded in mesoblastic derivates in the adults of all the higher triploblastica, retains with marvellous constancy its epiblastic origin (though it is usually separated from the epiblast prior to its histogenic differentiation); yet in the Cephalopoda, and some other Mollusca, the evidence is in favour of its developing in the mesoblast. Should future investigations confirm these conclusions, a good example will be afforded of an organ changing the layer from which it usually develops[130]. The explanation of such a change would be precisely the same as that already given for the mesoblast as a whole.
The actual mode of origin of various tissues, which in the true triploblastic forms arise in mesoblast, can be traced in the Cœlenterata[131]. In this group the epiblast and hypoblast both give rise to muscular and connective-tissue elements; and although the main part of the nervous system is formed in the epiblast, it seems certain that in some types nerves may be derived from the hypoblast[132]. These facts are extremely interesting, but it is by no means certain that any conclusions can be directly drawn from them as to the actual origin of the mesoblast in the triploblastic forms, till we know from what diploblastic forms the triploblastica originated. All that they shew is that any of the constituents of the mesoblast may have originated from either of the primitive layers.
Fig. 207. Two transverse sections through embryos of Hydrophilus piceus. (After Kowalevsky.)
A. Section through an embryo at the point where the two germinal folds most approximate.
B. Section through an embryo, in the anterior region where the folds of the amnion have not united.
gg. germinal groove; me. mesoblast; am. amnion; yk. yolk.
For further light as to the origin of the mesoblast, it is necessary to turn to its actual development.
