DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 121 



upper lip and at the same time fuses with the nasal processes. This fusion 

 obliterates the naso-optic furrow and shuts off the connection between the 

 nasal pit and mouth slit (Figs. 98 and 99). Internally the concomitant 

 formation of the palate separates the oral and nasal cavities. The lateral 

 nasal process gives rise to the wing of the nose. The nose at first is a broad, 

 flat structure but later becomes elevated, elongated and narrower. The 

 lower jaw, lower lip and chin are formed by the mandibular processes. At 

 first the chin is short vertically but broad transversely. Later it becomes 

 longer and a transverse furrow divides the middle portion into the lower lip 

 and chin. 



The Extremities. 



The rudiments of the extremities or limbs appear in human embryos 

 about the end of the third week. They arise as small rounded protuberances 

 on the lateral body wall, the upper limb buds just caudal to the level of the 

 cervical flexure and the lower opposite the sacral flexure (Fig. 86) . The first 

 to appear are the upper buds, the lower appearing a little later. The differ- 

 ence in time of appearance is reflected through embryonic and fcetal life in 

 the slight advance in the degree of development of the upper over the lower. 



During the fourth week the limb buds become elongated, and a transverse 

 constriction on each bud marks off a distal from a proximal portion. The 

 proximal portion remains approximately cylindrical while the distal portion 

 becomes flat like a paddle (Fig. 88) . The flattened end-piece is the beginning 

 of the hand (or foot), which is thus the first part of the extremity to be differ- 

 entiated. During the sixth week the proximal portion, which in the mean- 

 time has become still longer, is marked off by a bend at the elbow (or knee) 

 into two segments, the fore-arm and arm (or leg and thigh). 



During the fifth week the rudimentary digits (fingers and toes) appear 

 on the flattened distal segments of the developing extremities. They are 

 first indicated by radial depressions on the flat surface, the digits being the 

 elevations between the depressions (Figs. 89 and 90). The digits grow 

 rapidly in thickness and length thus producing an apparent deepening of the 

 interdigital grooves and also forming protrusions around the distal free 

 borders of the limb-buds (Fig. 90). The depressed areas comprise a series 

 of membranes resembling the web in the feet of certain aquatic animals. 

 The digits grow more rapidly than the web, the latter eventually being con- 

 fined to the proximal part of the interdigital spaces. After the seventh week 

 the angle between the thumb and second digit approaches a right angle; a 

 lesser degree of approach to a right angle holds true in case of the great toe 

 (Figs. 91, 92 and 93). 



As the limbs first elongate their long axes lie nearly parallel with the long 



