46 



HUMAN EMBKYOLOGY. 



and the fourth, called the copula, formed by the conjoined ventral ends of the second 

 arches, is separated from the tuberculum impar by the orifice of the thyreoid 

 rudiment (Fig. 61). 



The two lateral elevations on the mandibular arches unite to form the greater 

 part of the ventral or anterior two-thirds of the tongue, upon which all the papillae 



Mandibular rudiments 



/ Tuberculum 

 impar 



Foramen 



Furcula 



Aperture of larynx 



Labiodental sulcus 



Mandibular rudiment 

 Tuberculum impar | 

 Hyoid rudiment 



Germ and 

 subst. - 

 eburnea 

 Foramen ^ 

 caecum 



Precervical sinus 

 Aperture of larynx 



FIG. 61. SCHEMA showing stages in the development of the tongue. 



are developed. The tuberculum impar either disappears or it forms the median 

 part of the anterior two-thirds of the organ. The posterior or dorsal third of 

 the tongue, which lies in the ventral or anterior wall of the permanent pharynx, 

 is formed from the copula of the second arches. It follows from what has been said 



Rudiment of respiratory system 

 Notochord 



Medulla spinalis 

 Ectoderm of embryo ^^*\ 



Rudiment of 

 thyreoid gland 



Hypophysi 



Rathke's pouch 



Ectoderm of amnion / 

 Mesoderm of amnion 



Cerebral 

 hemisphere 



I 

 Pericardium 



Liver diverticulum branching in septum transversum 



,' Stomach 



Dorsal pancreas rudiment 



Ventral pancreas rudiment 

 "V. Peritoneal part of coelum 



x^,. Caecum 



Peritoneal part of 

 coelum 



kWolffian duct 



Rectum 



r Tail gut 



Genito-urinary 

 chamber 



Cloacal membrane 

 Allantoic 

 diverticulum 



Chorion 



v ( Umbilical cord 

 ^ Placental mesoderrn 

 Yolk-sac 

 Septum transversum 



I 



FIG. 62. SCHEMA showing further stages in the development of the diverticula from the primitive gut and 

 modifications of the mid-gut and the mid-gut regions. The heart is not shown. (After Mall, modified. ) 



that the commencement of the thyreoid rudiment, which persists in the adult as 

 the foramen csecum of the tongue, must lie at the junction of the dorsal third with 

 the ventral two-thirds. In many cases it appears to lie in the dorsal end of the 

 ventral two-thirds, a position which may be associated with the fact that, in some 

 cases, the rudiment of the thyreoid passes through the substance of the tuberculum 



