Twenty Mongolian sheep embryos at the stage of 21 days and 16 hours, 7mm in length, were studied systematically and the following characteristic features were observed. The body of the 7mm sheep embryos looks like the letter“C”. It has approximately 44 pairs of somites encircling the notochord to form a cylindrical-like rod, located at the ventral side of the spinal cord. Four pairs of branchial arches have fully developed. The heart-thorax prominence projected higher than that of the liver-abdomen prominence. The fore limb bud is paddle-like, and the hind limb bud is cumulus-form. The brain has differentiated into five regions with three flexures. And the olfactory pits, optic cups and otic vesicles have also formed. Besides the tail nerves, most of the spinal nerves have grown and the brachial and lumbo-sacral plexuses have been formed, but the latter is not so conspicuous. The two plexuses extend into the bases of the limb buds. Of the 12 crainal nervers only the 1st, 2nd and 6th pairs have not yet developed. The nerve fibers of the Froriep’s ganglions extend into the N. hypoglossus. The Rathke’s pouch extends as a long canaliculus from the dorsal wall of the ectodermal mouth cavity. The paired lateral swellings arise from the mandibular arches. The transient tuberculum impar is located between the first and second branchial arches. The opening of the thyroglossal duct on the anterior border of the 2nd arches is closed. The cumulum epiglotticum develops from the bottom of the third branchial arches. The slitlike glottis of the larynx is located in the midplane between the 4th. and 6th branchial arches. The stomach bud expands into an oblong shape and the entire stomach has so rotated that the original dorsal border has turned 45?to the left. The intestinal loop has become V-shaped, and has extended into the umbilical cord. The gall bladder, the ventral and dorsal pancreas have developed. The terminal of the trachea bifurcates into the primary branchi. The vesicles of the lung buds have formed from the branchia of both sides. The single pericardial cavity, the paired pleural cavities, and the large common peritoneal cavities communicate with each other. The mesonephros are spindle-shaped complex. The course of the mesonephric ducts can be traced into the allantoic stalk. The metanephros is formed as a tubuloalveolar dilatation. The metanephric ducts communicates with the posterior dorsal border of the mesonephric ducts. The septum primum is going to fuse with the endocardial cushions, closing the iuteratrial foramen primum. A new opening, the interatrial foramen secondum, has been formed and the septum secondum is being developed. The interventricular foramen has been formed. The arterial bulbus is separated incompletely by the two longitudinal folds. The dorsal aortae between the third and the fourth aortic arches have not yet degenerated. The buddings of the coeliac artery, the posterior menenteric artery, the external iliac artery, and the inferior vena cava are not conspicuous. The other venous rudiments have already developed. Special venous circles have been discovered in the posterior part of the posterior cardinal vein and at the base of hind limb buds, the umbilical vein have also been found and sending branches to the bases of the posterior limb buds.

The cleavages and formation of the blastula of Mongolian sheep embryos were studied. The following observations were recorded.1. The egg of Mongolian sheep undergoes two maturation divisions. During the first maturation division the egg cell has a diameter of 136 ?. The cytoplasm contains larger or smaller yolk granules. The nucleus is located by one side of the cytoplasm. The nucleolus is larger and prominent. About 25 hours 30 minutes after coitus, after the second maturation division the egg cells are located at about distal 1/3 of the oviduct, and meassuring about 180 ? in diameter. The corona radiata desintegrates. The second polar body is located between vitelline membrane and zona pellucida.2. The cleavage of the Mongolian sheep embryos is of the equal holoblastic type. The first cleavage takes place about 44 hours 35 minutes after coitus. The blastomeres are about 104 ? in diameter. About 49 hours 50 minutes after coitus, the second cleavage completes the 4 blastomeres frequently lie at rght angles to each other, the diameter is about 108 ?. The 10-cell stage is attained about 66 hours after coitus, the diameter of the blastula is about 104 ?. Mean while the blastula moves slowly half way in the oviduct.3. About 68～74 hours after coitus, the number of blastomeres reached 16～20, and the diameter is about 108 ?, and it is located in about proximal 1/3 of the oviduct. Because of the shifting of the blastomeres during cleavage they form a solid mass of cells with a cleft appearing in the cellular mass, begins to assume the outline of the morula. From 135 hours 40 minutes to 139 hours 15 minutes, the number of the blastomeres is over 36, and the morula is about 100 ? in diameter. They are now considerably different in size, the largest blastomere being almost twice as the size of the smallest. By this time the morula has reached the uterus.4. About 160 hours 20 minutes to 188 hours 30 minutes after coitus, the blastula is about 100～108 ? in diameter, and is freely moving in the uterus. On the section of the blastula, a flat cell has been discovered from the lower surface of embryonic mass. It is considered as a primary cell to form the endoderm blastocytes and is named as "endodermoblast".

The Mongolian sheep embryo develops into gastrula from 8 days 17 hours 10 minutes to 12 days 14 hours 30 minutes after coitus. The blastocyst is spherical in shape about 266—972?m in diameter. The zona pellucida has disappeared. The trophoblast consists of simple flat epithlium or simple cuboidal epithelium. About 8 days 17 hours 10 minutes to 9 days 15 hours 18 minutes after coitus the embryonic knot becomes spheroid, about 70—72?m in diameter. The embryonic knot is slightly protruding from The surface of the blastocyst and is covered by the trophoblast, the ectodermal cells of the dorsal portion of the embryonic knot form a mass. The lower surface of the embryonic knot has differentiated into the endoderm. The extraembryonic endoderm extends to the inner surface of the trophoblast from the periphery of the embryoblast. About 10 days 16 hours to 10 days 17 hours 20 minutes after coitus, in the dorsal portion of the embryonic knot forms a vesicle which is known as the primitive amniotic cavity about 40?m in diameter. The bottom of the primitive amniotic cavity has differentiated into the ectoderm, about 72—90?m in diameter. The endoderm of the lower surface of the embryonic knot continues to develop along the inner surface of the trophoblast to the equator. The developmental process of the primitve amniotic cavity of Mongolian sheep embryos is different from rabbit、guinea pig and human embryos, but it is similar to the mole embryos. About 11 days 16 hours 5 minutes to 12 days 14 hours 30 minutes after coitus, the embryonic knot becomes discal in shape and is fully developed, about 252—396?m in diameter. The primitive amniotic cavity of the dorsal portion of the embryonic disc has disappeared. The ecitoderm is exposed to the surface of the blastocyst, it consists of stratified columnar epthelium. The endoderm of the lower surface of the embryonic disc consists of the simple flat epithelium, it forms a spherical primitive gut cavity along the inner surface of the trophoblbst.