r>r>{\ THE HUMAN EMBBYO. 



At the anterior end of the mesobranchial area, between the 

 ventral ends of the mandibular and hyoid arches, is a small 

 rounded elevation, the tuberculum impar (Fig. 237, tu). Behind 

 this, and between the ventral ends of the first and second 

 branchial arches, there is a much larger elevation, with prominent 

 rounded margins and a median longitudinal furrow. This is 

 the furcula (Fig. 237, fl) ; and from it the epiglottis will be 

 developed at a later stage, while the median groove will become 

 the glottis. 



The furcula lies at first immediately behind the tuberculum 

 impar ; but in the early part of the fourth week (Fig. 238) the 

 two become separated by a transverse ridge, formed from the 

 ventral ends of the hyoid and first branchial arches, which unite 

 together and extend across the floor of the mouth. This ridge 



/- Tu 



Fig. 242. — The tongue and floor of the mouth of a Human Embryo at the end 

 of the second month. (From His.) 



BE.l, first branchial arch. FK, foramen cascum. HY, hyoid arch. LT, glottis. 

 MN, mandibular arch. TU, body of the tongue, formed from the tuberculum impar. 



soon grows forwards at the sides of the tuberculum impar, 

 embracing it like a V. At the angle of the V, between the 

 ridge and the tuberculum, a small backwardly directed pit is 

 formed, the mouth of which becomes the foramen caecum (Fig. 

 239, fk), while the pit itself becomes the median portion of the 

 thyroid body, TH. 



The median part of the transverse ridge soon becomes 

 marked off by lateral grooves, and fusing with the tuberculum 

 impar gives rise to the root of the tongue (Fig. 242). The 

 V-shaped groove, marking the boundary between the two 

 originally separate elements of which the tongue consists, is 

 very conspicuous throughout development, and is often well 



