Embryology of the sheep. III. The respiratory system, mesenteries and celom in the fourteen to thirty‐four day embryo

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Abstract
The day‐by‐day developmental changes which occur in the respiratory system, mesenteries and celom of the sheep embryo from 14 to 34 days coital age are documented chronologically and described. This work provides standards of normal development which can be used as a basis for future work on abnormal development of these structures.The respiratory diverticulum which first appears on the seventeenth day is seen as a primitive lung bud on the nineteenth day and becomes bilobed on day 20. By the twenty‐first day a laryngotracheal tube is present and the larynx develops in the next ten days.The dorsal mesogastrium forms on the eighteenth day and the omental bursa is seen as an invagination by the twenty‐first day. A splenic primordium associated with the omentum is present by the twenty‐seventh day.The dorsal mesentery appears on the eighteenth day and undergoes torsion, herniation and coiling on the twentieth, twenty‐second and twenty‐seventh days. The ventral mesentery is present on the seventeenth day but disappears centrally between the eighteenth and twenty‐first days.An extraembryonic celom is seen on the fourteenth day and the formation of the intraembryonic celom occurs on day 15. Development of the pleuroperitoneal membranes commences on the twenty‐first day. Complete separation of pericardial and pleural cavities is completed by the twenty‐eighth day.