THE SYSTEMATIC MORPHOLOGICAL STUDY OF MONGOLIAN SHEEP EMBRYOS AT THE AGE OF 21 DAYS AND 16 HOURS / 解剖学报

ABSTRACT

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.