The uterine ova of Ascaris suum (Swine ascaris) were subjected to ordinary microscopic observation, cytochemical investigation, polarization and phase contrast microscopy as well as to an electron microscopic study. It was found that in fertilized ova there existed specific granules which were arranged underneath the inner surface of the egg membrane, and examination was made as to the fact that they had to do with the formation of chitin membrane of the egg shell and the following results were obtained.1. In ovarial ova, the granules already exist but they are not arranged in order.2. Immediately after fertilization these granules are arranged in a line, in contact with the inner surface of the egg membrane, and later they are agglutinated to each other. At the region 10(1)-9(3) of the uterus, the granules dwindle and, flattening, stick closely to the inner side of the egg membrane. When eggs arrive at the region 7, the egg membrane increases in thickness rapidly.3. In unfertilized ova, the granules are coarse, irregular in shape and scattered about, existing in the cell till ovulation. In the ova in the lower uterine region, findings were obtained which gave the author an impression that the granules themselves were probably changing into chitin membrane.4. Where the granules stick to the inner side of the egg membrane, there is seen an additional formation of its second layer, and stainability of the granules coincides with that of the chitin membrane. And it is often found that the surface layer of the stuck granules and the second layer show the same stainability.5. In the egg membrane of unfertilized ovum, the chitin substance, which is stained light blue by MANN's staining, seldom appears, but on the surface of the ovum, as in the case of fertilized ovum, there is an addition of albuminous membrane from the inside of the uterus.6. The fact that glycogen particles gather around the granules shows the possibility of synthesis of N-acetylglucosamine (which constitutes chitin) or of its precursor from glycogen.7. By phase contrast and electron microscopy, the findings which support the pre-mentioned findings were obtained. Especially, the endoplasmic reticulum, which showed complex appearance around nucleus, surrounded yolk granules and specific granules, directly adhering closely to the inner side of the egg membrane. Into the inside of the specific granules, the E. R. entered and made a complex network structure, and it could be supposed that in the inside of these granules absorption or synthesis of some substance is probably done. Further, although the E. R. surrounding the specific granules directly adhered closely to the inner side of the egg membrane, examination of the egg shell which was formed to some extent showed that it was made of many lamellae consisting of fine particles which coincide with the contour of this closely adhered E. R.8. From the above mentioned findings, the process of the formation of the egg membrane will be given by the following formula. E. R. Nucleus→Yolk granules→Glycogen-→Specific granules→E. R.→Chitin membrane The reason why mitchondria is not given in this diagram is because, as HORIE also stated, it was seldom found in the ovum of this stage and did appear in a large quantity only when yolk granules rapidly disappeared near the region 6. On the appearance of mitochondria an electron microscopic study is in progress as a separate subject.9. The author could prove the fact that the shell membrane of ascaris ovum is composed of chitin substance, in accordance with SCHULZE.