1046 



EMBRYO LOQT. 



Fig. 582. 



A9 



FIRST APPEARANCE OF THE LUNGS. 



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

 termination of bronchus in a very young 



Pig- 



Development of the Respiratory Apparatus. 



Observers -are not unanimous as to the development of the lungs. According 

 to Reichert and BischoflF, 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 ramifying- cavities (by the deliquescence or fusion 



of the internal cells), which communicate 

 with the trachea. Costa states that they 

 commence by a median, bud-like, hollow 

 process that opens into the oesophagus. 

 The walls of the communicating aperture 

 elongate considerably, and at a later period 

 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 

 ^''g- 583. same manner at the pharyngeal open- 



ing. It is always somewhat unde- 

 fined during youth, and its definitive 

 I* volume is not acquired until the 



period of puberty. 



The thymus gJand 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 



EMBRYO OF DOG, rWENTV-FIVE DAYS AFTER LAST 

 COPULATION. 



0, a, Nostrils ; h, b, eyes ; c, c, first visceral arches, 

 forming the lower jaw ; d, d, second visceral 

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

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

 liver, between the two lobes of which is seen the 

 divided orifice of the omphalo-mesenteric vein; I, 

 stomach ; m, intestine, communicating with the 

 umbilical vesicle, n n ; o o, corpora Wolffiana; pp, 

 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 



