THE ORGANS OF THE INNER GERM-LAYER. 



321 



Soon the groove-like evagination becomes separated from the over- 

 lying portion of the alimentary tube by two lateral ridges ; this 

 furnishes the first indication of a differentiation into oesophagus and 

 trachea (fig. 181). Then there grow out from the enlarged posterior 

 ends of the groove (figs. 181, 163) two small sacs (Lg) toward the two 

 sides of the body (in the Chick iu the middle of the third day), the 

 fundaments of the right 

 and left lung. Enveloped 

 in a thick layer of em- 

 bryonic connective 

 tissue, they are in im- 

 mediate contact behind 

 with the fundament of 

 the heart; laterally they 

 project into the anterior 

 fissure-like prolongation 

 cf the body - cavity. 

 With this the essential 

 parts of the respiratory 

 apparatus are estab- 

 lished; at this stage 

 in amniotic Vertebrates 

 they resemble the simple 

 sac-like structures which 

 the lungs of Amphibia 

 present permanently. 



In the further course 

 of development the fun- 

 daments of trachea and 

 oasophagus, which com- 

 municate by means of a 

 fissure, become separated 

 by a constriction which 

 begins behind, where the 

 pulmonary sacs have budded out, and gradually moves forward. The 

 constricting off is here interrupted at the place which becomes the 

 entrance to the larynx. The latter is distinguishable in the case of 

 Man at the end of the fifth week as an enlargement at the beginning 

 of the fundament of the trachea. It acquires its cartilages in the 

 eighth or ninth week. Of these the thyroid cartilage arises, according 

 to the comparative-anatomical investigations of DUBOIS, from a fusion 



21 



Fig. 181. Alimentary tube of a human embryo (R of His) 

 5 mm. long, neck measurement. From His, ' ' Mensch- 

 liche Embryonen." Magnified 20 diameters. 



RT, RATHKE'S pouch ; Uk, lower jaw ; Sd, thyroid gland ; 

 Ch, chorda dorsalis ; Kk, entrance to the larynx ; 

 Lg, lung ; Mg, stomach ; P, pancreas ; Lbg, primitive 

 hepatic duct ; Ds, vitelline duct (stalk of the intestine) ; 

 All, allantoic duct; W, Wolffian duct, with kidney- 

 duct (ureter) budding out of it ; B, bursa pelvis. 



