250 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



4th arch, bounds the pulmonary groove ; in its anterior part, 

 which is the most prominent, is developed the epiglottis ; its 

 lateral margins, which bound the pulmonary groove, form the 

 aryteno-epiglottic folds 



tuberculum impar. 



The margin of the groove ^thus be- 



1st recess (saliuary glands) 

 2nd recess (tonsil) 

 median thyroid bud. 

 .furcula (epiglottis) 

 3rd recess (thymus) 

 4th recess flat, thyroid bud) 



aryteno. epi. fold 



—coetom (pericardium) 



pulmonary groove 



stomach 



Fie. 204. — Floor of the Pharynx and Oesophagus of a human embryo of 3 weeks 

 showing; the Furcula, Pulmonary Groove, and Diverticulum. (After His.) 



comes the upper aperture of the larynx. Two points should 

 be noted in connection with the relationships of the oesophagus 

 at the 3rd week: (1) like that of a fish, it is extremely short; 

 (2) it lies between the right and left cavities of the coelom 

 (Figs. 204 and 205) in the dorsal attachment of mesocardium 

 of the sinus venosus (Fig. 202). (3) The part of the coelom 

 which lies at each side of the oesophagus, is the narrow isthmus 

 connecting the pericardial and peritoneal cavities which after- 

 wards become the pleurae (Figs. 201 and 205). 



When the pulmonary bud or groove is viewed from the side, 

 its posterior extremity is seen to end in a deep pocket, the 

 pulmonary pocket or diverticulum (Fig. 22, p. 30). The wall 

 of the pocket is lined by a mass of hypoblast, which ulti- 

 mately forms the epithelial lining of the whole respiratory 

 tract, from the ciliated epithelium of the trachea to the pavement 

 epithelium lining the alveoli of the lungs. Eound the pulmonary 

 bud is grouped a mass of mesoblastic tissue out of which the con- 

 nective-tissue system of the trachea, bronchi and lungs is developed. 



