1. In this communication is described for the first time the unsegmented germinal disc of the monotreme Echidna (Tachyglossus), and measurements are also given of living intra-uterine eggs at early stages.
2. Segmentation is not initiated in Echidna until the egg has arrived in the uterus.
3. The polar bodies are much smaller than those described for Ornithorhynchus. The first is much larger than the second but, contrary to what has been reported to occur in Ornithorhynchus, remains undivided.
4. The germ nuclei, immediately before the formation of the first cleavage spindle, undergo conjugation and fusion.
5. Contrary to what is described for Ornithorhynchus, polyspermy does not normally occur in the fertilization of the ovum of Echidna.