1046 



EMBRYOLOGY 



9 



iillST APPEARANCE OF THE LUNGS. 



In a Fowl at four days ; b, at six days ; c, 

 terfliiuation of bronchus in a very young 



Pig- 



Development of the Respiratory Apparatus. 



Observers are not unanimous as to the development of tlie limc/s. According 

 to Reichert and Bischotf, they arise from two small solid cellular masses lying on 

 the surface of the anterior portion of the intestinal canal. These become 

 channeled out into numerous ramifyino- cavities (by the deliquescence or fusion 



of the internal cells), which communicate 



^'g ^82. ^yjtjj ^jjg ti-auhea. Costa states that they 



M^ j^^k ^H^ commence by a median, bud-like, hollow 



MJm iIFa ■ ^^^^^ process that opens into the cesophairus. 



■ Im 'wm ^|^9/j^3 The walls of the communicating- aperture 



■iA9 mW ^m^^ elongate considerably, and at a later period 



^^^ ^w *" form the trachea and larynx ; while the 



hollow bud divides into two pyriform sacs, 

 each of which becomes broken up into a 

 multitude of subdivisions to constitute 

 the pulmonary lobes, with their vesicles 

 and infundibula. 



The trachea is completed by the development of the cartilaginous rings in the 

 tube that binds the lungs to the oesophagus. They appear at the commencement 



of the third month. 



The larynx is developed in the 

 Fig- -^83. same manner at the pharyngeal open- 



ing. It is always somewhat unde- 

 fined during youth, and its definitive 

 volume is not acquired until the 

 period of puberty. 



The tinjmus gland appears as a 

 process of the respiratory mucous 

 membrane. It seems to be formed 

 at the larynx, and gradually descends 

 along the trachea to the entrance of 

 the thorax. 



Development of the Digestive 

 Apparatus. 



In this paragraph, the develop- 

 ment of the alimentary canal will 

 be first studied, then that of the 

 organs annexed to it. 



A. Alimentary Canal. — "We 

 have seen how the embryo, in be- 

 coming incurvated, divides the blasto- 

 dermic vesicle into two parts which 

 communicate by a large pedicle. The 

 external portion is the umbilical 



EMBRVO OF DOG, TWKNTV-FIVE DAYS AFTER LAST 

 COPULATION. 



o, a, Nostrils ; 6, h, eyes ; c, c, first visceral arches, 

 forming the lower jaw ; rf. d, second visceral 

 arches ; e, right auricle ; /, left auricle ; g, right 

 ventricle; A, left ventricle; i, aortic bulb; k k, 

 liver, between the two lobes of whu-h is seen the 

 /livided orifice of the omphalo-mesentoric vein ; /, 

 stomach; w, intestine, communic.iting with the 

 umbilical vesicle, ?» n; o o, corpora Woltfiana ; p p, 

 allantois ; q, q, anterior extremities ; r, r, posterior, 

 extremities. 



vesicle ; the pedicle is the omphalo- 

 mesenteric duct, and the inner part the intestinal cavity. 



The latter may be resolved into three portions : the anterior intestine, which 



