I study development of thymus in white rat, including 30 embryos of 12-21 days, the 10 new-born΄s (1st day) and 20 rats of 1-3 month, on serial histological sections, staining by hematoxylin and eosin, azur-II and eosin, and total preparations. Definitive thymus is the association of two embryonic thymuses, right and left. These 2 classic lobes of thymus are false lobes which form secondary unpaired organ. Earch of the 2 classic lobes is divided on the true lobes. I find many true lobes of thymus with different sizes, about 4 rigft and 4 left - cranial, middle, caudal and dorsolateral. The cranial lobe narrows in cranial direction and can rise up on the neck along trachea. In situ thymus has shape of bulb or cone, but after preparation it looks like pitchfork. False lobes of thymus remind the deformed cones.
The paired anlage of thymus is determined about embryonic neck. Lympho-epithelial tapes of right and left thymuses grow in caudal direction and descend into thoracic cavity on 15th day of the rat embryogenesis. Caudal ends of the thymuses lean against base of heart and widen on the both sides. In result magistralizating thymic branches of internal thoracic arteries sink into the right and left thymuses with their division on the cranial, middle and caudal true lobes. Internal thoracic artery gives off pericardiacophrenic artery. It and soname vein with phrenic nerve passes in caudal direction moreover in ventral side from root of lung. These bundle separates small dorsolateral secondary lobe from each caudal lobe of thymus in white rat of second month old. Lungs limit the lateral growth of the thymic right and left false lobes. Therefore they go round the nerve-vascular longitudinal bundles from dorsal and then from lateral side.
The work was submitted to the International Scientific Conference «Basic research», Croatia, 28 July - 1 August, 2012, came to the editorial office 29.06.2012.