Fig. 208. Figures illustrating the development of Astacus. (From Parker; after Reichenbach.)
A. Section through part of the ovum during segmentation. n. nuclei; w.y. white yolk; y.p. yolk pyramids; c. central yolk mass.
B. and C. Longitudinal sections of the gastrula stage. a. archenteron; b. blastopore; ms. mesoblast; ec. epiblast; en. hypoblast, distinguished from epiblast by shading.
D. Highly magnified view of anterior lip of blastopore, to shew the origin of the primary mesoblast from the wall of the archenteron. p.ms. primary mesoblast; ec. epiblast; en. hypoblast.
E. Two hypoblast cells to shew the amœeba-like absorption of yolk spheres. y. yolk; n. nucleus; p. pseudopodial process.
F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.ms.); n. nucleus.
The following summary illustrates the more important modes in which the mesoblast originates.
1. It grows inwards from the lips of the blastopore as a pair of bands. In these cases it may originate (a) from cells which are clearly hypoblastic, (b) from cells which are clearly epiblastic, (c) from cells which cannot be regarded as belonging to either layer.
Mollusca.—Gasteropoda, Cephalopoda, and Lamellibranchiata. In Gasteropoda and Lamellibranchiata the mesoblast sometimes originates from a pair of cells at the lips of the blastopore, though very probably some of the elements subsequently come from the epiblast; and in Cephalopoda it begins as a ring of cells round the edge of the blastoderm.
Polyzoa Entoprocta.—It originates from a pair of cells at the lips of the blastopore.
Chætopoda.—Euaxes. It arises as a ridge of cells at the lips of the blastopore ([fig. 200]).
Gephyrea.—Bonellia. It arises ([fig. 206]) as an infolding of the epiblastic lips of the blastopore.
Nematelminthes.—Cucullanus. It grows backwards from the hypoblast cells at the persistent oral opening of the blastopore.
Tracheata.—Insecta. It grows inwards from the lips of the germinal groove ([fig. 207]), which probably represent the remains of a blastopore. Part of the mesoblast is probably also derived from the yolk-cells. A similar though more modified development of the mesoblast occurs in the Araneina ([fig. 214]).
Crustacea.—Decapoda. It partly grows in from the hypoblastic lips of the blastopore, and is partly derived from the yolk-cells ([fig. 208]).