Observations on the development of Ornithorhynchus

The paper treats of certain stages in the intrauterine development of the egg of Ornithorhynchus. The stages dealt with include the following:— (1)An early (eight-celled) stage of segmentation. (2)A stage manifesting a later phase of the segmentation-process. (3)A stage of early germinal-layer formation, in which the cellular blastoderm is almost exclusively arranged in the form of a much-attenuated epithelial membrane covering part of the yolk-mass. The arrangement on the whole resembles that illustrated in fig. 22, Taf. 8, and fig. 33, Taf. 9, in R. Semon’s work on “Monotreme Development.”* Certain differences are, however, noticeable, more especially as regards the absence of the deeply placed nuclei figured by Semon.