THE CLEAVAGES AND FORMATION OF THE BLASTULA OF MONGOLIAN SHEEP EMBRYOS / 解剖学报

ABSTRACT

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".