228 



EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



The stomodasum appears shortly after stage E, on the anterior margin of the 

 procephalic lobes, its inner end coming to lie in the midst of this cloud of degen- 

 erating nuclei. (Fig. 140, G.) After stage /, no trace of these yolk nuclei is visible. 



I have pointed out that a similar condition exists in Acilius and other insects, 

 and that it is probably characteristic of arthropods in general. This infolding is 

 the only one in arthropods that, in respect to the time of its appearance, its loca- 

 tion, and its products, can be regarded as a gastrula, and we shall designate it as 

 such. 



. 



The Germ Wall. 



The germ wall is a narrow, unsegmented zone of proliferating cells, first seen 

 along the margins of the germinal disc, and later along the sides of the embryo up 

 to the last stage in the closing of the haemal surface, g.w. 



The pairs of large nuclei in stage A (Fig. 123, p.bl.), located on the margins 

 of the germinal area, mark the beginnings of the germ wall and of the periblast. 



m.l. P r& - ^ 



FIG. 128. Sections of the germinal area of Limulus in stage D, showing the formation of the inner layers, 

 and the extension of the germinal area by the growth of the germ wall, g.w., over the surface of the yolk. The 

 local inward proliferation of the blastoderm to form, in part, the mesoblastic somites, is shown at a.\ m., median 

 line. Sections Da and DC from the procephalic, and anterior thoracic, regions. Section D&. from the region 

 of the primitive streak, pr.s. 



At these points an inward proliferation develops, in surface views appearing as 

 faint spots or depressions; later they form a conspicuous marginal band or thick- 

 ening. (Figs. 125, 128, 140, g.w.) 



The inward proliferation along this margin is similar to that at the apex of 

 the body. As the germinal margin spreads over the surface of the yolk, it leaves 

 behind a trail of differentiated ectoderm, mesoderm, and yolk cells, or periblast. 

 It ceases to produce new periblast after stage /, but it continues to proliferate the 

 definitive ectoderm and new ectoderm, probably up to the time the cephalic navel 

 closes. 



