Fig. 90. Section of the germinal disc of a Fowl during the later stages of segmentation.
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.
There is hardly any question in development which has been the subject of so much controversy as the mode of formation of the germinal layers in the common Fowl. The differences in the views of authors have been caused to a large extent by the difficulties of the investigation, but perhaps still more by the fact that many of the observations were made at a time when the methods of making sections were very inferior to those of the present day. The subject itself is by no means of an importance commensurate with the attention it has received. The characters which belong to the formation of the layers in the Sauropsida are secondarily derived from those in the Ichthyopsida, and are of but little importance for the general questions which concern the nature and origin of the germinal layers. In the account in the sequel I have avoided as much as possible discussion of controverted points. My statements are founded in the main on my own observations, more especially on a recent investigation carried on in conjunction with my pupil, Mr Deighton. It is to Kölliker (No. [135]), and to Gasser (No. [127]) that the most important of the more recent advances in our knowledge are due. Kölliker, in his great work on Embryology, definitely established the essential connection between the primitive streak and the formation of the mesoblast; but while confirming his statement on this head, I am obliged to differ from him with reference to some other points.
Gasser’s work, especially that part of it which relates to the passages leading from the neural to the alimentary canal, which he was the first to discover, is very valuable.
The blastoderm gradually grows in size, and extends itself over the yolk; the growth over the yolk being very largely effected by an increase in the size of the area opaca, which during this process becomes more distinctly marked off from the area pellucida. The area pellucida gradually assumes an oval form, and at the same time becomes divided into a posterior opaque region and an anterior transparent region. The posterior opacity is named by some authors the embryonic shield.