THE FCETUS. 



919 



during youth, and its definitive volume is not acquired until the period of 

 puberty. 



The tJiymus gland appears as a process of the respiratory mucous mem- 

 brane; it seems to be formed at the 



larynx, and gradually descends along Fi S- 446 - 



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 becoming 

 incurvated, divides the blastodermic vesicle into two parts which com- 

 municate by a large pedicle. The external portion is the umbilical vesicle ; 

 the pedicle is the omphalo-mesenteric duct, and the inner part the intestinal 

 cavity. 



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



Fig. 447. 



FIRST APPEARANCE OF THE LUNGS ; 

 In a Fowl at four days ; 6, At six days ; c, 

 Termination of bronchus in a very young 



EMBRYO OF DOG, TWENTY-FIVE DAYS AFTER LAST COPULATION. 

 , a, Nostrils ; 6, 6, Eyes ; c, c, First visceral arches, forming the lower jaw ; d, d, 

 Second viscei'al arches ; <?, Right auricle ; /, Left auricle ; g, Right ventricle ; 

 A, 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 ; p, Al- 

 lantois ; q, q, Anterior extremities ; r, r, Posterior extremities. 



which forms the pharynx and oesophagus ; the posterior intestine, that gives 

 rise to the rectum ; and the middle intestine, which becomes the stomach 

 and intestines. 



