. THE FIFTH WEEK. 



497 



half of the entire embryo. The shape of the head is still deter- 

 mined almost entirely by the brain, of which the several divisions 

 are clearly recognisable from the surface. All parts of the brain 

 increase considerably in 

 size, and more especially 

 the cerebral hemispheres. 



During the fifth week 

 the face is gradually ac- 

 quiring definite form, and 

 the features are becoming 

 established. 



The olfactory pits deepen 

 considerably ; and their 

 inner and outer borders 

 become raised into pro- 

 minent lips. The inner 

 borders are formed by the 

 lateral margins of the 

 fronto-nasal process, which 

 grow out as two rounded 

 wings, the processus globu- 

 lares (Figs. 206, 207, ro). The outer borders are formed by 

 the lateral frontal processes, which separate the olfactory pits 

 from the eyes. 



The lower margin of each olfactory pit is incomplete, and 

 between the processus globularis and the lateral frontal process 

 there is a deep nasal groove (Fig. 206), leading from the olfactory 

 pit to the stomatodasum. Towards the end of the fifth week, 

 the maxillary arches (Fig. 207, MX) become more prominent, and 

 growing inwards meet the processus globulares, FO, and fuse with 

 these ; thus bridging over the nasal grooves, and converting 

 them into short tubes, the posterior narial passages, which lead 

 from the olfactory pits to the mouth. At the same time the 

 apertures of the olfactory pits become narrowed, to form the 

 external nostrils. 



The bridge of the nose is formed from the median part 

 the fronto-nasal process (Figs. 206, 207, FP). At the com- 

 mencement of the fifth week this is a triangular area, slightly 

 depressed below the level of the surrounding parts ; but towards 

 the close of the week, a blunt process appears in the centre of 



K K 



FlG. 205. Human Embryo lettered by 

 Professor His, Rg, and estimated as 

 thirty-two or thirty-three days old. 

 (From His.) x 5. 



