Fig. 2. Surface-view of a germinal disc with two furrows.
Figs. 3, 4, 5. Surface-views of three germinal discs in different stages of segmentation.
Fig. 6. Section through the germinal disc represented in fig 3. _n._ nucleus; _x._ edge of germinal disc. The engraver has not accurately copied my original drawings in respect to the structure of the segmentation furrows.
Figs. 6_a_ and 6_b_. Two furrows of the same germinal disc more highly magnified.
Fig. 6_c_. A nucleus from the same germinal disc highly magnified.
Fig. 7. Section through a germinal disc of the same age as that represented in fig. 4. _n._ nucleus; _nx._ modified nucleus; _nx´._ modified nucleus of the yolk; _f._ furrow appearing in the yolk around the germinal disc.
Figs. 7_a_, 7_b_, 7_c_. Three segments with modified nuclei from the same germinal disc.
Fig. 8. Section through a somewhat older germinal disc. _ep._ epiblast; _n´._ nuclei of yolk.
Figs. 8_a_, 8_b_, 8_c_. Modified nuclei from the yolk from the same germinal disc.
Fig. 8_d_. Segment in the act of division from the same germinal disc.
Fig. 9. Section through a germinal disc in which the segmentation is completed. It shews the larger collection of cells at the embryonic end of the germinal disc than at the non-embryonic. _ep._ epiblast.
_c._ Cells formed in the yolk around the nuclei of the yolk. _ep._ Epiblast. _er._ Embryonic ring. _es._ Embryo swelling. _hy._ Hypoblast. _ll._ Lower layer cells. _ly._ Line separating the yolk from the blastoderm. _m._ Mesoblast. _mg._ Medullary groove. _n´._ Nuclei of yolk. _na._ Cells to form ventral wall of alimentary canal which have been derived from the yolk. _nal._ Cells formed around the nuclei of the yolk which have entered the hypoblast. _sc._ Segmentation cavity. _vp._ Combined lateral and vertebral plate of mesoblast.
Fig. 1. Longitudinal section of a blastoderm at the first appearance of the segmentation cavity.
Fig. 2. Longitudinal section through a blastoderm after the layer of cells has disappeared from the floor of the segmentation cavity. _bd._ Large cell resting on the yolk, probably remaining over from the later periods of segmentation. Magnified 60 diameters. (Hardened in chromic acid.)
The section is intended to illustrate the fact that the nuclei form a layer in the yolk under the floor of the segmentation cavity. The roof of the segmentation cavity is broken.