42 



HUMAN EMBEYOLOGY. 



fold round a pit-like hollow called the proctodseum (Fig. 60), which is separated 



from the caudal part of the ento- 

 smodiai trophobiast dermal portion of the alimentary 

 canal, until about the fourth 



ellular trophoblast 



Fore-gut Notpchord 

 Amiiion cavity^ 



Neural tube " 



Ccelomv 



a 



Mesodermofchorion anal jg^^gg^^ a portion Of the 



fammon more extensive cloacal mem- 



aiuniou 

 >- Body stalk 



brane mentioned on p. 39. 



Peri 

 cardium 



Hind-gut 



Wall of yolk-sa 



FIG. 55. SAGITTAL SECTION OF ZYGOTE SHOWN IN FIG. 38. 



Differentiation of the 

 Fore-gut. 



Derivatives of the Lateral 

 Wall. Shortly after the fore- 

 gut is enclosed, and whilst it is 

 still separated from the stomato- 

 daeum by the bucco-pharyngeal 

 membrane, its cephalic extremity 

 dilates to form the primitive 

 pharynx and thereafter, a series 

 of eight pouches are formed in its 

 walls, five in each lateral wall ; the 

 pharyngeal or branchial pouches ; 

 two in its ventral wall, one near 



the cephalic extremity, the rudiment of the thyreoid gland, and a second situated 

 more caudally, which is the germ of the respiratory system, that is, of the larynx, 

 the trachea, the bronchi, and the epithelial lining of the lungs. The eighth pouch, 

 Seessel's pouch, is formed in the dorsal wall, immediately caudal to the dorsal end of the 

 bucco-pharyngeal membrane, and it projects into the floor of the primitive cranium. 



Ext. ear{ 



1st Branchial 

 cleft 



2nd Branchial 

 pouch 



2nd Branchial. } 

 cleft 



3rd Branchial 

 pouch " 



4th Branchial 



pouch - 



4th Branchial .. 

 cleft 



Pharyngo-branchial duct, 

 |5th Branchial bar / 



5th Branchial pouch 



Separating membrane / / 



- 1st cleft = tympanum / / 

 and tube V 



Tonsil 



Lower Upper- 



- parathyreoid parathyreoid 



Upper Lower -_^ 



-parathyreoid parathyreoid 



.Thyreoid gland ' 



**" Thymus 



- - Hyoid bone 



- - Thyreo-glossal duct 

 Thyreoid cartilage 



\ 



Thymus 



FIG. 56. SCHEMA showing the branchial pouches, the branchial clefts, the branchial bars, and the thyreo- 

 glossal duct and some of their derivatives. I., II., III., IV., and V., the five branchial bars. 



Simultaneously with the formation of the pharyngeal pouches internally a 

 series of clefts appear externally. They correspond in position with the first four 

 pharyngeal pouches, and they are called the pharyngeal or branchial clefts. 



By means of the pharyngeal pouches and clefts the lateral boundary of the 

 cephalic part of the fore-gut, on each side, is divided into a series of bars, the 

 pharyngeal or branchial bars, five in number, but the fifth is distinctly visible only 

 in the inner aspect of the pharynx. 



