ANATOMY OF A 17.8 MM. HUMAN EMBRYO 35 



occurred. The line of fusion between the first and second bran- 

 chial arches is marked ventrally by a transverse groove, dorsal 

 to which is seen the fossa conchae. The grooves between the 

 other branchial arches have disappeared. 



The limb buds extend nearly perpendicularly to the longitu- 

 dinal axis of the body. The upper project slightly beyond the 

 ventral border of the body, and show a differentiation of arm, 

 forearm, and clearly outlined digits. The latter protrude slightly 

 beyond the border of the hand-plate. Upon the lower limb 

 buds are slight indications of developing toes. 



Circular thickenings of the epidermis on the lateral body walls 

 mark the developing mammae. In section these thickenings ap- 

 pear slightly convex on the surface, and project into the under- 

 lying mesenchyma. The umbilical cord as it leaves the body 

 wall bends towards the right. 



DIGESTIVE SYSTEM 



Oral cavity. The primitive oral cavity appears in median 

 sagittal section in plate 1, but is represented more fully in plate 

 4, a portion of the tongue having been cut away. It is a short, 

 dorso-ventrally compressed passage consisting of a roof and a floor, 

 the epithelia of which meet laterally at the angle of the mouth. 

 The external aperture of the primitive oral cavity, the rima oris, 

 (R.or.) is indicated as seen from the exterior in plates 2, 3 and 

 5; as seen in median sagittal section in plates 1 and 4. It is a 

 narrow, horizontal expanded orifice, concave dorsally where it is 

 bounded by the fused maxillary and median nasal processes, 

 convex ventrally where it is formed by the united mandibular 

 processes. 



Dorsally the oral cavity communicates with the pharynx, the 

 division between the two being marked in the median line on the 

 roof by the stalk of the hypophysis (Hyp.) While the roof, as 

 thus bounded, is of considerable extent, the floor is very lim- 

 ited, consisting of merely the anlage of the lower lip and teeth. 



The lip-grooves are just beginning to indent the oral epithe- 

 lium. The lower lip-groove is seen in sagittal section in plate 1. 



