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. [FIG. 92. TRANSVERSE SECTION THROUGH THE BLASTODERM OF A CHICK BEFORE THE APPEARANCE OF THE PRIMITIVE STREAK. The epiblast is represented somewhat diagrammatically. The hyphens shew the points of junction of the two halves of the section.] During these changes the epiblast (fig. 92) becomes two layers deep over the greater part of the area pellucida, though still only one cell deep in the area opaca. The irregular hypoblast spheres of the unincubated blastoderm flatten themselves out, and unite into a definite hypoblastic membrane (fig. 92). Between this membrane and the epiblast there remain a number of scattered cells (fig. 92) which cannot however be said to form a definite layer altogether distinct from the hypoblast. They are almost entirely confined to the posterior part of the area pellucida, and give rise to the opacity of that part.