GERMINAL LAYERS OF THE EMBRYO. 
265 
Gephyrea. — Bonellia. It arises (fig. 9) as an infolding 
of the epiblastic lips of the blastopore. 
Fig 11. — Figures illustrating the Development of Astacus. (From Parker, 
after Reichenbacb.) A. Section through part of the ovum during seg- 
mentation. n. nuelei ; w.y. white yolk ; yp. yolk pyramids ; c. central 
yolk mass. B and C. Longitudinal sections of the gastrula stage. 
a. archenteron; blastopore ; mesoblast; ec. epiblast ; hypo- 
blast, distinguished from epiblast by shading. D. Highly magnified 
view of anterior lip of blastopore, to show 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 show the amoeba- 
like absorption of yolk spheres, y. yolk ; n. nucleus ; g. pseudopodial 
process. F. Hypoblast cells giving rise endogenously to the secondary 
mesoblast {sms?) ; n. nucleus. 
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. 10), 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. IT). 
Crustacea. — Decapoda. It partly grows in from the hypo- 
blastic lips of the blastopore, and is partly derived from the 
yolk-cells (fig. 11). 
2. The mesoblast is developed from the walls of hollow 
