In the preceding paper on the milk-spots of omentum of rabbit, I concluded that in the mesothelial cell-layer of the milk-spots there exist the peculiar epithelioid cells (histiocytes) and apertures, and also affirmed that the majority of the epithelioid cells are penetrating the mesothelial cell-layer to regain their position in the milk-spots from the peritoneal cavity. From this fact, I cannot agree with the views of Renaut and Goldmann-"Diese Zelle (Histiozyt) wandert nicht aus der serösen Höhlen in das Netz, um hier tâches laiteuses aufzubauen, die Wanderung ist vielmehr eine umgekehrte". Elaborating upon my view, the mother cells which bring forth the milk-spots also may be considered to be the similar cells reappeared as above mentioned. To investigate the above subject, this study was experimentally done. In addition, the developmental process of the four groups of milk-spots (Follicular, adventitial, mixed and solitary form by the author) was also investigated. Concerning to the development of the omentum, there are two important questions hitherto discussed, namely: the one is the formation of the net-meshes in omentum: the other is Ranvier's cellules vasoformatives et sanguiformatives, and these matters were also some illustrated in this paper. However, the latter question, relating to the system of the blood-vessels was here limited to its objective observation only and its detail will be carried out in another paper. Conclusions:- 1. The mother cells of the milk-spots may probably be those cells which once emigrated from the peritoneum into the abdominal cavity and then deposit on the surface of the omentum. 2. The mother cells are grouped or disseminated on the surface of the omentum and form the milk-spots, having depositted deeper and deeper into the omental tissue. However, the adveventitial milk-spots may be built partly by the adventitia-cells which have existed embryonally in the omentum. 3. The milk-spots will be developped by means of mitotic cell-division of the mother cells and by the continuous deposition of the large mononuclear wandering cells from the abdominal cavity. 4. In the beginning of the development of milk-spots, all of them, excluding the adventitial form, belong to the solitary form, and after a little while some of the solitary form are supplied with blood-vessels and consequently they become the follicular form. The follicular form developed along the relatively large bloog-vessel will become a mixed form, combining with the adventitial form. 5. Considering the origin of the milk-spots, I cannot quite agree with Renaut's classification hitherto used. 6. The formation of the net-meshes in omentum of rat is originated from the necrosis or falling of the nuclei of the mesothelial cells and Auerbach's "Schaltplättchen".