Previously the author undertook the investigation on the early stages of development of the heart of the birds and published the results. On the later stages of development of the heart, the mode of formation of the interatrial septum is very complicated so that many investigators failed to reach the same conclusion hither-to-fore. Consequently Prof. Shikinami gave me the problem on the first appearance and the further developmental stages of the interatrial septum, especially its fenestration using the pig embryos. First an attempt was made to ascertain if the reports published in the literature on the formation of the interatrial septum is correct or not. As the material, the pig embryos were used. A large majority of them were fixed in Zenker's fluid, and stained with borax carmine. After the subsequent treatment in the usual manner, these specimen were embedded in paraffin, trimmed and cut in transverse serial sections with a thickness of ten microns. A number of models were prepared according to Born-Peter's method of wax-plate reconstruction. From the results of my experiments, the following summary and conclusions may be made: 1) The interatrial septum is first formed in an embryo 45mm. in greatest length with 23-24 somites on the vault of the atrium in the median line, and it is a little more eminent on the ventral than on the dorsal side. 2) The interatrial septum is first formed by the cessation of the development of the heart wall along the median line, and then it grows up to the atrial canal by the actual protrusion of the part formed by the endothelial proliferation. 3) The interventricular septum is first formed in an embryo 4, 0mm. in greatest length with 24-25 somites. 4) The interatrial septum is first perforated with several small openings (interatrial foramen secundum) in an embryo 6.0mm. in length with 45 somites. 5) These openings increase in both their numbers and sizes, and are confluent in a single opening at first in an embryo 7.0mm. in length. 6) The closure of the interatrial foramen primum is completed in an embryo 6.5mm. in length. It is a little later than the first appearance of the interatrial foramen secundum. 7) The partition of the chambers of the heart except the interatrial septum is accomplished in an embryo 17.0mm. in length.