1) The coelom appering early before the formation of somite in hunran embryo remains simple without division till the horizon XVI developmental stage.
2) In this earlier stages the pericardial cavity becomes very large, while the pleuroperitoneal part remains small, as the development of the digestive tube is much retarded to that of the heart.
3) In the horizon XVI the pleuropericardial ductus becomes narrow to the canal between the Ductus Cuvieri and tracheal swelling. The latter is the mesenchym surrounding the both sides bronchi at their proximal parts.
4) In that horizon XVI the formation of cartilage begins at the occipital basal plate, cervical vertebral bodies and proceeds to the lateral part of the occipital on the one hand, to the thoracic vertebral bodies on the other. In the arm and the leg the humerus, radius, ulna, femur and tibia are cartilageous at the end of this stage. N. phrenicus reaches septum transversum in the beginning of this stage.
5) The narrrow pleuropericardial canal in the horizon XVI obliteriert soon in the next horizon XVII on the left side at first, then on the right.
6) In this horizon XVII the cartilageous change proceeds to the sphenoidal part, to the vestibular part of the otic capsule, to the ribs, to the scapula of the arm, to the fibula of the leg. The formation of the three semicircular canales in the vestibular part of the otic vesicle is also proceeding in this stage, that of the lateral semicircular is the last among them at the end of this stage.
7) For the division of the pleural from the peritoneal cavity the so-called pulmonal ridges on the dorsal and lateral body wall protrude caudal along the surface of the inferior lobe of the lung and develope under the lung ventral and lateralward so that it surrounds ventral with the sepsum transversum the communicating opening between these two cavities, at the ventrolateral and caudal part of pleural cavity.
8) The communicating opening between the two cavities becomes smaller during the horizon XVII so that it becornes a 160μ long at the right, 220μ long at the left narrow opening in out 18mm (a) long embryo, a 225μ long at the right, 125μ long at the left narrow one in our 20mm long specimen.
9) The obliteration of this narrow pleuroperitoneal canal seems to occur at the left side at first, then at the right.
10) After Mall or Broman the oblitaration of the right pleuroperitoneal canal occurs earlier than the left, which conclusion depends perhaps upon a scanty material.
11) In the horizon XVIII stage there appear ala orbitalis. ala temporalis, proc. ali-cochlearis, Meckel's and Reichert's cartilages in the chondrocranium, three semicircular canals and 6 peripheral nerves ending to the epithelium of the otic vesicle are all complete.

View full abstract