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. EXPLANATION OF PLATE 7. (X. p. 246.) COMPLETE LIST OF REFERENCE LETTERS. _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. Fig. 2_a_. Portion of same blastoderm highly magnified, to shew the characters of the nuclei of the yolk _n´_ and the nuclei in the cells of the blastoderm. Fig. 2_b_. Large knobbed nucleus from the same blastoderm, very highly magnified. Fig. 2_c_. Nucleus of yolk from the same blastoderm. Fig. 3. Longitudinal section of blastoderm of same stage as fig. 2. (Hardened in chromic acid.) Fig. 4. Longitudinal section of blastoderm slightly older than fig. 2. Magnified 45 diameters. (Hardened in osmic acid.) It illustrates (1) the characters of the epiblast; (2) the embryonic swelling; (3) the segmentation cavity. Fig. 5. Longitudinal section through a blastoderm at the time of the first appearance of the embryonic rim, and before the formation of the medullary groove. Magnified 45 diameters. Fig. 5_a_. Section through the periphery of the embryonic rim of the blastoderm of which fig. 5 represents a section. Fig. 6. Section through the embryonic rim of a blastoderm somewhat younger than that represented on Pl. 8, fig. B. Fig. 7. Section through the most projecting portion of the embryonic rim of a blastoderm of the same age as that represented on Pl. 8, fig. B. The section is drawn on a very considerably smaller scale than that on fig. 5. It is intended to illustrate the growth of the embryonic rim and the disappearance of the segmentation cavity.