During the fifth day remarkable changes take place in the epiblast of the embryonic area. It is probable that its constituent cells increase in number and become one by one columnar; and that in the process they press against the layer of rounded elements below them, so that the two layers cease to be distinguishable, and the whole embryonic area acquires in section the characters represented in [fig. 136][79]. Towards the end of the sixth day the embryonic area becomes oval, but the changes which next take place are not understood. In the front part of the area only two layers of cells are found, (1) an hypoblast, and (2) an epiblast of columnar cells probably derived from the flattened epiblast cells of the earlier stages. In the posterior part of the blastoderm a middle layer is present (Van Beneden) in addition to the two other layers; and this layer probably originates from the middle layer which extended throughout the area at the beginning of the fifth day, and then became fused with the epiblast. The middle layer does not give rise to the whole of the eventual mesoblast, but only to part of it. From its origin it may be called the hypoblastic mesoblast, and it is probably equivalent to the hypoblastic mesoblast already described in the chick (pp. 154 and 155). The stage just described has only been met with by Van Beneden[80].
Fig. 136. Section through the nearly circular embryonic area of a Rabbit’s ovum of six days, nine hours and .8 mm. in diameter.
The section shews the peculiar character of the upper layer with a certain number of superficial flattened cells; and represents about half the breadth of the area.
A diagrammatic view of the whole blastodermic vesicle at about the beginning of the seventh day is given in [fig. 137]. The embryonic area is represented in white. The line ge in B shews the extension of the hypoblast round the inner side of the vesicle. The blastodermic vesicle is therefore formed of three areas, (1) the embryonic area with three layers: this area is placed where the blastopore was originally situated. (2) The ring around the embryonic area where the walls of the vesicle are formed of epiblast and hypoblast. (3) The area beyond this again where the vesicle is formed of epiblast only[81].
Fig. 137. Views of the blastodermic vesicle of a Rabbit on the seventh day without the zona. A. from above, B. from the side. (From Kölliker.)
ag. embryonic area; ge. boundary of the hypoblast.
The changes which next take place begin with the formation of a primitive streak, homologous with, and in most respects similar to, the primitive streak in Birds. The formation of the streak is preceded by that of a clear spot near the middle of the blastoderm, forming the nodal point of Hensen. This spot subsequently constitutes the front end of the primitive streak.
The history of the primitive streak was first worked out in a satisfactory manner by Hensen (No. [182]), from whom however I differ in admitting the existence of a certain part of the mesoblast before its appearance.