It may be fairly objected to our theory, as Prof. Geikie himself has suggested, that the high angle at which the strata dip is difficult to account for. But, in fact, this steep inclination constitutes the very difficulty which any hypothesis on the subject must be framed to explain; and it is a difficulty which is not more easily solved by Prof. Geikie's theory than by our own. II. THE DEVELOPMENT AND GROWTH OF THE LAYERS OF THE BLASTODERM[6]. Footnote 6: From the _Quarterly Journal of Microscopical Science_, Vol. XIII., 1873. With Plate 1, figs. 1-5 and 9-12. The following paper deals with the changes which take place in the cells of the blastoderm of the hen's egg during the first thirty or forty hours of incubation. The subject is one which has, as a general rule, not been much followed up by embryologists, but is nevertheless of the greatest interest, both in reference to embryology itself, and to the growth and changes of protoplasm exhibited in simple embryonic cells. I am far from having exhausted the subject in this paper, and in some cases I shall be able merely to state facts, without being able to give any explanation of their meaning. My method of investigation has been the examination of sections and surface views. For hardening the blastoderm I have employed, as usual, chromic acid, and also gold chloride. It is, however, difficult to make sections of blastoderms hardened by this latter reagent, and the sections when made are not in all cases satisfactory. For surface views I have chiefly used silver nitrate, which brings out the outlines of the cells in a manner which leaves nothing to be desired as to clearness. If the outlines only of the cells are to be examined, a very short immersion (half a minute) of the blastoderm in a half per cent. solution of silver nitrate is sufficient, but if the immersion lasts for a longer period the nuclei will be brought out also. For studying the latter, however, I have found it better to employ gold chloride or carmine in conjunction with the silver nitrate. My observations begin with the blastoderm of a freshly laid egg. The appearances presented by sections of this have been accurately described by Peremeschko, "Ueber die Bildung der Keimblätter im Hühnerei," _Sitzungsberichte der K. Akademie der Wissenschaften in Wien_, 1868. Oellacher, "Untersuchung über die Furchung und Blatterbildung im Hühnerei," _Studien aus dem Institut für Experim. Pathologie in Wien_, 1870 (pp. 54-74), and Dr Klein, lxiii. Bande der _Sitz. der K. Acadamie der Wiss. in Wien_, 1871.