The section, which represents rather more than half the breadth of the blastoderm (the middle line being shewn at _c_), shews that the upper and central parts of the disc segment faster than those below and towards the periphery. At the periphery the segments are still very large. One of the larger segments is shewn at _a_. In the majority of segments a nucleus can be seen; and it seems probable that the nucleus is present in them all. Most of the segments are filled with highly refracting spherules, but these are more numerous in some cells (especially the larger cells near the yolk) than in others. In the central part of the blastoderm the upper cells have commenced to form a distinct layer. No segmentation cavity is present. _a._ large peripheral cell; _b._ larger cells of the lower parts of the blastoderm; _c._ middle line of blastoderm; _e._ edge of the blastoderm adjoining the white yolk; _w._ white yolk.] [FIG. 91. SECTION OF A BLASTODERM OF A FOWL'S EGG AT THE COMMENCEMENT OF INCUBATION. The thin epiblast _ep_ composed of columnar cells rests on the incomplete lower layer _l_, composed of larger and more granular hypoblast cells. The lower layer is thicker in some places than in others, and is especially thick at the periphery. The line below the under layer marks the upper surface of the white yolk. The larger so-called formative cells are seen at _b_, lying on the white yolk. The figure does not take in quite the whole breadth of the blastoderm; but the reader must understand that both to the right hand and to the left _ep_ is continued farther than _l_, so that at the extreme edge it rests directly on the white yolk.] As a result of incubation the blastoderm undergoes a series of changes, which end in the definite formation of three germinal layers, and in the establishment of the chief systems of organs of the embryo. The more important of these changes are accomplished in the case of the common Fowl during the first day and the early part of the second day of incubation.