782 DEVELOPMENT OF THE ABDOMIXAL GLAXUS. 



liver, ^vith respect to the exact share of the hypoblastic and mesoblastic 

 elements in the formation of the glandular cells. The main duct and its 

 branches undoubtedly owe their origin to diverticula proceeding from 

 the intestinal hypoblast, and the epithelial lining of the ducts is doubt- 

 less derived from that source. By those who consider that the glan- 

 dular cells also arise from the hypoblast, solid processes of that layer 

 are described as stretching into the mass of mesoblast. Into these the 

 diverticular cavities subsequently extend in more than one main divi- 

 sion. The blood-vessels and the connective tissue of the gland are 

 undoubtedly due to the mesoblastic elements, and these are very soon 

 combined with the imrts proceeding from hypoblast. 



The Spleen. — This organ appears soon after the pancreas as a 

 thickening of the mesogastrium, and is therefore entirely mesoblastic 

 in its origin. (Peremeschko, Vienna Acad., 18G7, and W. Mliller in 

 Strieker's Histol.) The gradual formation of the trabecular structure 

 from one set of cells and of the blood-vessels and cellular elements of 

 the organ from tlie blastemic substance has been traced. The pulp is 

 formed in connection with the veins, and the arteries are formed along 

 with the Malpighian corpuscles. The spleen is closely related to the 

 pancreas in its origin, but it is later of being formed and contains no 

 hypoblastic elements. In the human foetus of about twelve weeks its 

 shape can be recognised, but the Malpighian bodies are not visible till 

 the middle of foetal life. 



Lymphatic Glands. — The development of the mesenteric lymphatic 

 glands has been observed by Sertoli in mammals. (Vienna Acad., 

 18G6.) The blastema from which they are produced is imbedded in 

 the mesentery, and is therefore entirely mesoblastic. The gradual 

 differentiation of the blastema gives rise in succession to the lymph 

 spaces, the trabecuke and the lymph cells, and the distinction follows 

 between inferent and efferent lymphatic vessels. The development of 

 lym])hatic vessels has been described in the General Anatomy, p. 191. 



The Thymus and Thyroid Glands.— The development of these 

 bodies has been described in an earlier part of this volume, pp. 297 

 and 299. The thymus gland proceeds entirely from mesoblastic tissue ; 

 but, according to the researches of W. Miiller (Jenaisch. Zeitsch., 

 1871), it would appear that the thyroid body arises at first as a diver- 

 ticulum from the pharynx, and it therefore contains some hypoblastic 

 elements. 



DEVELOPMENT OF THE LUNGS AND TRACHEA. 



The lungs first appear as two small protrusions upon the front of the 

 oesophageal jiortion of the alimentary canal, completely hid by the 

 rudimentary heart and liver. These primitive protrusions or tubercles 

 are visible in the chick on the third day of incubation, and in the 

 embryoes of mammalia and of man at a corresponding stage of advance- 

 ment. Their internal cavities communicate with the oesophagus, and 

 are lined by a prolongation of the hypoblast. At a later period they 

 are connected with the oesophagus by means of a long pedicle, which 

 ultimately forms the trachea, whilst the bronchia and air-cells are 

 developed by the progressive ramification of the internal cavity in the 

 form of cffical tubes, after the manner of the ducts of glands. 



The diverticulum of hypoblast is surrounded by a mass of meso- 

 blastic cells, so that the pulmonary parenchyma, like that of the glands, 



