1. The reduction division in the embryo sac mother cells of Ulmus hollandica belgica Rehd. does not reveal abnormal features. 2. The embryo sac is of the Lilium-type, with the restriction that the number of the antipodal cells has augmented. 3. Synergids and antipodal cells, as a rule, disappear during the later development. 4. The situation of the ovulum makes clear that the chalazogamic fertilization must be the ordinary one. 5. Within the tissue of stigma, style and integuments pollen tubes never could be demonstrated. 6. Only in two cases out of about 250 ovules studied a kind of chalazogamic intrusion by a ’’pollen tube” has been observed. 7. The development of the seed may take place in different ways: we ranged the cases observed into four categories (Chapter IV). .... 8. For the greater part the seeds show abnormalities and in those which are not abnormal, yet a lacking in correlation during their development can be observed. 9. Some cases of double embryos are described and their origin is discussed in chapter V. 10. It appears that owing to i° the fact that all the trees belonging to the U. hollandica belgica type are shedding their pollen at the same time and i° the non-simultaneous ripening of the pollen and the embryo sac, a fertilization by pollen of the same type is impossible. 11. The seeds that still develop must be regarded as possessing hybrid embryos: this, apparently, is the cause of the sterility in U. hollandica belgica.