File:Keibel Mall 2 609.jpg

Fig. 609 a and b. Part of a transverse section through the embryo 2.5 mm greatest length 13-14 pairs of primitive segments

Embryo Pfannenstiel III from the collection of the late Professor Pfannenstiel, Kiel.)

The section passes through the cloacal membrane and its exact position may be seen from the adjacent figure b. The mesoderm is still quite loosely aggregated. Between it and the entoderm there is at the upper edge of the figure a fully-formed portion of the rete peri-intestinale, and further down there are cell chains and masses (to the left of the cloaca) from which new networks of the rete are forming. The section shows at two places, to the right and left of the cloacal membrane, large cells partly multinucleate, partly filled with yolk granules and partly free from them, and lying free between the mesoderm and endoderm. These cells may be termed wandering cells and hypothetically may be interpreted as primary genital cells.